| Message ID | 20200604215236.2798244-1-eblake@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v4] osdep: Make MIN/MAX evaluate arguments only once | expand |
On 6/4/20 4:52 PM, Eric Blake wrote: > I'm not aware of any immediate bugs in qemu where a second runtime > evalution of the arguments to MIN() or MAX() causes a problem, but evaluation > proactively preventing such abuse is easier than falling prey to an > unintended case down the road. At any rate, here's the conversation > that sparked the current patch: > https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg05718.html >
On 6/4/20 2:52 PM, Eric Blake wrote: > I'm not aware of any immediate bugs in qemu where a second runtime > evalution of the arguments to MIN() or MAX() causes a problem, but > proactively preventing such abuse is easier than falling prey to an > unintended case down the road. At any rate, here's the conversation > that sparked the current patch: > https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg05718.html > > Update the MIN/MAX macros to only evaluate their argument once at > runtime; this uses typeof(1 ? (a) : (b)) to ensure that we are > promoting the temporaries to the same type as the final comparison (we > have to trigger type promotion, as typeof(bitfield) won't compile; and > we can't use typeof((a) + (b)) or even typeof((a) + 0), as some of our > uses of MAX are on void* pointers where such addition is undefined). > > However, we are unable to work around gcc refusing to compile ({}) in > a constant context (such as the array length of a static variable), > even when only used in the dead branch of a __builtin_choose_expr(), > so we have to provide a second macro pair MIN_CONST and MAX_CONST for > use when both arguments are known to be compile-time constants and > where the result must also be usable as a constant; this second form > evaluates arguments multiple times but that doesn't matter for > constants. By using a void expression as the expansion if a > non-constant is presented to this second form, we can enlist the > compiler to ensure the double evaluation is not attempted on > non-constants. > > Alas, as both macros now rely on compiler intrinsics, they are no > longer usable in preprocessor #if conditions; those will just have to > be open-coded or the logic rewritten into #define or runtime 'if' > conditions (but where the compiler dead-code-elimination will probably > still apply). > > I tested that both gcc 10.1.1 and clang 10.0.0 produce errors for all > forms of macro mis-use. As the errors can sometimes be cryptic, I'm > demonstrating the gcc output: > > Use of MIN when MIN_CONST is needed: > > In file included from /home/eblake/qemu/qemu-img.c:25: > /home/eblake/qemu/include/qemu/osdep.h:249:5: error: braced-group within expression allowed only inside a function > 249 | ({ \ > | ^ > /home/eblake/qemu/qemu-img.c:92:12: note: in expansion of macro ‘MIN’ > 92 | char array[MIN(1, 2)] = ""; > | ^~~ > > Use of MIN_CONST when MIN is needed: > > /home/eblake/qemu/qemu-img.c: In function ‘is_allocated_sectors’: > /home/eblake/qemu/qemu-img.c:1225:15: error: void value not ignored as it ought to be > 1225 | i = MIN_CONST(i, n); > | ^ > > Use of MIN in the preprocessor: > > In file included from /home/eblake/qemu/accel/tcg/translate-all.c:20: > /home/eblake/qemu/accel/tcg/translate-all.c: In function ‘page_check_range’: > /home/eblake/qemu/include/qemu/osdep.h:249:6: error: token "{" is not valid in preprocessor expressions > 249 | ({ \ > | ^ > > Fix the resulting callsites that used #if or computed a compile-time > constant min or max to use the new macros. cpu-defs.h is interesting, > as CPU_TLB_DYN_MAX_BITS is sometimes used as a constant and sometimes > dynamic. > > Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> r~
On Thursday, 2020-06-04 at 16:52:36 -05, Eric Blake wrote:
> /* Minimum function that returns zero only iff both values are zero.
Not your change, but "only" is unnecessary here (or iff -> if).
dme.
On 6/5/20 10:24 AM, David Edmondson wrote: > On Thursday, 2020-06-04 at 16:52:36 -05, Eric Blake wrote: > >> /* Minimum function that returns zero only iff both values are zero. > > Not your change, but "only" is unnecessary here (or iff -> if). Maybe we should teach checkpatch.pl to refuse the idiomatic 'iff', it is already hard enough for non-native English to read (or write) technical documentation...
On 6/5/20 3:24 AM, David Edmondson wrote: > On Thursday, 2020-06-04 at 16:52:36 -05, Eric Blake wrote: > >> /* Minimum function that returns zero only iff both values are zero. > > Not your change, but "only" is unnecessary here (or iff -> if). Indeed. Whoever picks this up could make that change (although I have no idea which tree will be best for this patch, so I may just adopt it as a miscellaneous patch through one of my own if there are no objections after a few days...).
On 04/06/20 23:52, Eric Blake wrote: > I'm not aware of any immediate bugs in qemu where a second runtime > evalution of the arguments to MIN() or MAX() causes a problem, but > proactively preventing such abuse is easier than falling prey to an > unintended case down the road. At any rate, here's the conversation > that sparked the current patch: > https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg05718.html > > Update the MIN/MAX macros to only evaluate their argument once at > runtime; this uses typeof(1 ? (a) : (b)) to ensure that we are > promoting the temporaries to the same type as the final comparison (we > have to trigger type promotion, as typeof(bitfield) won't compile; and > we can't use typeof((a) + (b)) or even typeof((a) + 0), as some of our > uses of MAX are on void* pointers where such addition is undefined). > > However, we are unable to work around gcc refusing to compile ({}) in > a constant context (such as the array length of a static variable), > even when only used in the dead branch of a __builtin_choose_expr(), > so we have to provide a second macro pair MIN_CONST and MAX_CONST for > use when both arguments are known to be compile-time constants and > where the result must also be usable as a constant; this second form > evaluates arguments multiple times but that doesn't matter for > constants. By using a void expression as the expansion if a > non-constant is presented to this second form, we can enlist the > compiler to ensure the double evaluation is not attempted on > non-constants. > > Alas, as both macros now rely on compiler intrinsics, they are no > longer usable in preprocessor #if conditions; those will just have to > be open-coded or the logic rewritten into #define or runtime 'if' > conditions (but where the compiler dead-code-elimination will probably > still apply). > > I tested that both gcc 10.1.1 and clang 10.0.0 produce errors for all > forms of macro mis-use. As the errors can sometimes be cryptic, I'm > demonstrating the gcc output: > > Use of MIN when MIN_CONST is needed: > > In file included from /home/eblake/qemu/qemu-img.c:25: > /home/eblake/qemu/include/qemu/osdep.h:249:5: error: braced-group within expression allowed only inside a function > 249 | ({ \ > | ^ > /home/eblake/qemu/qemu-img.c:92:12: note: in expansion of macro ‘MIN’ > 92 | char array[MIN(1, 2)] = ""; > | ^~~ > > Use of MIN_CONST when MIN is needed: > > /home/eblake/qemu/qemu-img.c: In function ‘is_allocated_sectors’: > /home/eblake/qemu/qemu-img.c:1225:15: error: void value not ignored as it ought to be > 1225 | i = MIN_CONST(i, n); > | ^ > > Use of MIN in the preprocessor: > > In file included from /home/eblake/qemu/accel/tcg/translate-all.c:20: > /home/eblake/qemu/accel/tcg/translate-all.c: In function ‘page_check_range’: > /home/eblake/qemu/include/qemu/osdep.h:249:6: error: token "{" is not valid in preprocessor expressions > 249 | ({ \ > | ^ > > Fix the resulting callsites that used #if or computed a compile-time > constant min or max to use the new macros. cpu-defs.h is interesting, > as CPU_TLB_DYN_MAX_BITS is sometimes used as a constant and sometimes > dynamic. > > Signed-off-by: Eric Blake <eblake@redhat.com> > > --- > > v2 was: https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00727.html > v3: avoid __auto_type [Richard], document other approaches that fail > [Dave], rebase to master > v4: use ((void)0) instead of __builtin_unreachable [Dave], update comments > --- > hw/usb/hcd-xhci.h | 2 +- > include/block/block.h | 4 +-- > include/exec/cpu-all.h | 8 +++--- > include/exec/cpu-defs.h | 7 +++++- > include/qemu/osdep.h | 51 +++++++++++++++++++++++++++++++++------ > accel/tcg/translate-all.c | 6 ++--- > migration/qemu-file.c | 2 +- > 7 files changed, 59 insertions(+), 21 deletions(-) > > diff --git a/hw/usb/hcd-xhci.h b/hw/usb/hcd-xhci.h > index 2fad4df2a704..946af51fc25d 100644 > --- a/hw/usb/hcd-xhci.h > +++ b/hw/usb/hcd-xhci.h > @@ -214,7 +214,7 @@ struct XHCIState { > uint32_t dcbaap_high; > uint32_t config; > > - USBPort uports[MAX(MAXPORTS_2, MAXPORTS_3)]; > + USBPort uports[MAX_CONST(MAXPORTS_2, MAXPORTS_3)]; > XHCIPort ports[MAXPORTS]; > XHCISlot slots[MAXSLOTS]; > uint32_t numports; > diff --git a/include/block/block.h b/include/block/block.h > index 25e299605e19..e8fc8149967f 100644 > --- a/include/block/block.h > +++ b/include/block/block.h > @@ -133,8 +133,8 @@ typedef struct HDGeometry { > #define BDRV_SECTOR_BITS 9 > #define BDRV_SECTOR_SIZE (1ULL << BDRV_SECTOR_BITS) > > -#define BDRV_REQUEST_MAX_SECTORS MIN(SIZE_MAX >> BDRV_SECTOR_BITS, \ > - INT_MAX >> BDRV_SECTOR_BITS) > +#define BDRV_REQUEST_MAX_SECTORS MIN_CONST(SIZE_MAX >> BDRV_SECTOR_BITS, \ > + INT_MAX >> BDRV_SECTOR_BITS) > #define BDRV_REQUEST_MAX_BYTES (BDRV_REQUEST_MAX_SECTORS << BDRV_SECTOR_BITS) > > /* > diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h > index d14374bdd499..291454fac12e 100644 > --- a/include/exec/cpu-all.h > +++ b/include/exec/cpu-all.h > @@ -176,11 +176,9 @@ extern unsigned long reserved_va; > * avoid setting bits at the top of guest addresses that might need > * to be used for tags. > */ > -#if MIN(TARGET_VIRT_ADDR_SPACE_BITS, TARGET_ABI_BITS) <= 32 > -# define GUEST_ADDR_MAX_ UINT32_MAX > -#else > -# define GUEST_ADDR_MAX_ (~0ul) > -#endif > +#define GUEST_ADDR_MAX_ \ > + ((MIN_CONST(TARGET_VIRT_ADDR_SPACE_BITS, TARGET_ABI_BITS) <= 32) ? \ > + UINT32_MAX : ~0ul) > #define GUEST_ADDR_MAX (reserved_va ? reserved_va - 1 : GUEST_ADDR_MAX_) > > #else > diff --git a/include/exec/cpu-defs.h b/include/exec/cpu-defs.h > index 8c44abefa22a..918563233797 100644 > --- a/include/exec/cpu-defs.h > +++ b/include/exec/cpu-defs.h > @@ -102,8 +102,13 @@ typedef uint64_t target_ulong; > * Skylake's Level-2 STLB has 16 1G entries. > * Also, make sure we do not size the TLB past the guest's address space. > */ > -# define CPU_TLB_DYN_MAX_BITS \ > +# ifdef TARGET_PAGE_BITS_VARY > +# define CPU_TLB_DYN_MAX_BITS \ > MIN(22, TARGET_VIRT_ADDR_SPACE_BITS - TARGET_PAGE_BITS) > +# else > +# define CPU_TLB_DYN_MAX_BITS \ > + MIN_CONST(22, TARGET_VIRT_ADDR_SPACE_BITS - TARGET_PAGE_BITS) > +# endif > # endif > > typedef struct CPUTLBEntry { > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h > index ff7c17b85735..b18584f32ff6 100644 > --- a/include/qemu/osdep.h > +++ b/include/qemu/osdep.h > @@ -236,18 +236,53 @@ extern int daemon(int, int); > #define SIZE_MAX ((size_t)-1) > #endif > > -#ifndef MIN > -#define MIN(a, b) (((a) < (b)) ? (a) : (b)) > -#endif > -#ifndef MAX > -#define MAX(a, b) (((a) > (b)) ? (a) : (b)) > -#endif > +/* > + * Two variations of MIN/MAX macros. The first is for runtime use, and > + * evaluates arguments only once (so it is safe even with side > + * effects), but will not work in constant contexts (such as array > + * size declarations) because of the '{}'. The second is for constant > + * expression use, where evaluating arguments twice is safe because > + * the result is going to be constant anyway, but will not work in a > + * runtime context because of a void expression where a value is > + * expected. Thus, both gcc and clang will fail to compile if you use > + * the wrong macro (even if the error may seem a bit cryptic). > + * > + * Note that neither form is usable as an #if condition; if you truly > + * need to write conditional code that depends on a minimum or maximum > + * determined by the pre-processor instead of the compiler, you'll > + * have to open-code it. > + */ > +#undef MIN > +#define MIN(a, b) \ > + ({ \ > + typeof(1 ? (a) : (b)) _a = (a), _b = (b); \ > + _a < _b ? _a : _b; \ > + }) > +#define MIN_CONST(a, b) \ > + __builtin_choose_expr( \ > + __builtin_constant_p(a) && __builtin_constant_p(b), \ > + (a) < (b) ? (a) : (b), \ > + ((void)0)) > +#undef MAX > +#define MAX(a, b) \ > + ({ \ > + typeof(1 ? (a) : (b)) _a = (a), _b = (b); \ > + _a > _b ? _a : _b; \ > + }) > +#define MAX_CONST(a, b) \ > + __builtin_choose_expr( \ > + __builtin_constant_p(a) && __builtin_constant_p(b), \ > + (a) > (b) ? (a) : (b), \ > + __builtin_unreachable()) > > /* Minimum function that returns zero only iff both values are zero. > * Intended for use with unsigned values only. */ > #ifndef MIN_NON_ZERO > -#define MIN_NON_ZERO(a, b) ((a) == 0 ? (b) : \ > - ((b) == 0 ? (a) : (MIN(a, b)))) > +#define MIN_NON_ZERO(a, b) \ > + ({ \ > + typeof(1 ? (a) : (b)) _a = (a), _b = (b); \ > + _a == 0 ? _b : (_b == 0 || _b > _a) ? _a : _b; \ > + }) > #endif > > /* Round number down to multiple */ > diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c > index 42ce1dfcff77..d77add79b218 100644 > --- a/accel/tcg/translate-all.c > +++ b/accel/tcg/translate-all.c > @@ -2565,9 +2565,9 @@ int page_check_range(target_ulong start, target_ulong len, int flags) > /* This function should never be called with addresses outside the > guest address space. If this assert fires, it probably indicates > a missing call to h2g_valid. */ > -#if TARGET_ABI_BITS > L1_MAP_ADDR_SPACE_BITS > - assert(start < ((target_ulong)1 << L1_MAP_ADDR_SPACE_BITS)); > -#endif > + if (TARGET_ABI_BITS > L1_MAP_ADDR_SPACE_BITS) { > + assert(start < ((target_ulong)1 << L1_MAP_ADDR_SPACE_BITS)); > + } > > if (len == 0) { > return 0; > diff --git a/migration/qemu-file.c b/migration/qemu-file.c > index 1c3a358a140d..be21518c5708 100644 > --- a/migration/qemu-file.c > +++ b/migration/qemu-file.c > @@ -31,7 +31,7 @@ > #include "qapi/error.h" > > #define IO_BUF_SIZE 32768 > -#define MAX_IOV_SIZE MIN(IOV_MAX, 64) > +#define MAX_IOV_SIZE MIN_CONST(IOV_MAX, 64) > > struct QEMUFile { > const QEMUFileOps *ops; > Queued, thnaks. Paolo
diff --git a/hw/usb/hcd-xhci.h b/hw/usb/hcd-xhci.h index 2fad4df2a704..946af51fc25d 100644 --- a/hw/usb/hcd-xhci.h +++ b/hw/usb/hcd-xhci.h @@ -214,7 +214,7 @@ struct XHCIState { uint32_t dcbaap_high; uint32_t config; - USBPort uports[MAX(MAXPORTS_2, MAXPORTS_3)]; + USBPort uports[MAX_CONST(MAXPORTS_2, MAXPORTS_3)]; XHCIPort ports[MAXPORTS]; XHCISlot slots[MAXSLOTS]; uint32_t numports; diff --git a/include/block/block.h b/include/block/block.h index 25e299605e19..e8fc8149967f 100644 --- a/include/block/block.h +++ b/include/block/block.h @@ -133,8 +133,8 @@ typedef struct HDGeometry { #define BDRV_SECTOR_BITS 9 #define BDRV_SECTOR_SIZE (1ULL << BDRV_SECTOR_BITS) -#define BDRV_REQUEST_MAX_SECTORS MIN(SIZE_MAX >> BDRV_SECTOR_BITS, \ - INT_MAX >> BDRV_SECTOR_BITS) +#define BDRV_REQUEST_MAX_SECTORS MIN_CONST(SIZE_MAX >> BDRV_SECTOR_BITS, \ + INT_MAX >> BDRV_SECTOR_BITS) #define BDRV_REQUEST_MAX_BYTES (BDRV_REQUEST_MAX_SECTORS << BDRV_SECTOR_BITS) /* diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h index d14374bdd499..291454fac12e 100644 --- a/include/exec/cpu-all.h +++ b/include/exec/cpu-all.h @@ -176,11 +176,9 @@ extern unsigned long reserved_va; * avoid setting bits at the top of guest addresses that might need * to be used for tags. */ -#if MIN(TARGET_VIRT_ADDR_SPACE_BITS, TARGET_ABI_BITS) <= 32 -# define GUEST_ADDR_MAX_ UINT32_MAX -#else -# define GUEST_ADDR_MAX_ (~0ul) -#endif +#define GUEST_ADDR_MAX_ \ + ((MIN_CONST(TARGET_VIRT_ADDR_SPACE_BITS, TARGET_ABI_BITS) <= 32) ? \ + UINT32_MAX : ~0ul) #define GUEST_ADDR_MAX (reserved_va ? reserved_va - 1 : GUEST_ADDR_MAX_) #else diff --git a/include/exec/cpu-defs.h b/include/exec/cpu-defs.h index 8c44abefa22a..918563233797 100644 --- a/include/exec/cpu-defs.h +++ b/include/exec/cpu-defs.h @@ -102,8 +102,13 @@ typedef uint64_t target_ulong; * Skylake's Level-2 STLB has 16 1G entries. * Also, make sure we do not size the TLB past the guest's address space. */ -# define CPU_TLB_DYN_MAX_BITS \ +# ifdef TARGET_PAGE_BITS_VARY +# define CPU_TLB_DYN_MAX_BITS \ MIN(22, TARGET_VIRT_ADDR_SPACE_BITS - TARGET_PAGE_BITS) +# else +# define CPU_TLB_DYN_MAX_BITS \ + MIN_CONST(22, TARGET_VIRT_ADDR_SPACE_BITS - TARGET_PAGE_BITS) +# endif # endif typedef struct CPUTLBEntry { diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h index ff7c17b85735..b18584f32ff6 100644 --- a/include/qemu/osdep.h +++ b/include/qemu/osdep.h @@ -236,18 +236,53 @@ extern int daemon(int, int); #define SIZE_MAX ((size_t)-1) #endif -#ifndef MIN -#define MIN(a, b) (((a) < (b)) ? (a) : (b)) -#endif -#ifndef MAX -#define MAX(a, b) (((a) > (b)) ? (a) : (b)) -#endif +/* + * Two variations of MIN/MAX macros. The first is for runtime use, and + * evaluates arguments only once (so it is safe even with side + * effects), but will not work in constant contexts (such as array + * size declarations) because of the '{}'. The second is for constant + * expression use, where evaluating arguments twice is safe because + * the result is going to be constant anyway, but will not work in a + * runtime context because of a void expression where a value is + * expected. Thus, both gcc and clang will fail to compile if you use + * the wrong macro (even if the error may seem a bit cryptic). + * + * Note that neither form is usable as an #if condition; if you truly + * need to write conditional code that depends on a minimum or maximum + * determined by the pre-processor instead of the compiler, you'll + * have to open-code it. + */ +#undef MIN +#define MIN(a, b) \ + ({ \ + typeof(1 ? (a) : (b)) _a = (a), _b = (b); \ + _a < _b ? _a : _b; \ + }) +#define MIN_CONST(a, b) \ + __builtin_choose_expr( \ + __builtin_constant_p(a) && __builtin_constant_p(b), \ + (a) < (b) ? (a) : (b), \ + ((void)0)) +#undef MAX +#define MAX(a, b) \ + ({ \ + typeof(1 ? (a) : (b)) _a = (a), _b = (b); \ + _a > _b ? _a : _b; \ + }) +#define MAX_CONST(a, b) \ + __builtin_choose_expr( \ + __builtin_constant_p(a) && __builtin_constant_p(b), \ + (a) > (b) ? (a) : (b), \ + __builtin_unreachable()) /* Minimum function that returns zero only iff both values are zero. * Intended for use with unsigned values only. */ #ifndef MIN_NON_ZERO -#define MIN_NON_ZERO(a, b) ((a) == 0 ? (b) : \ - ((b) == 0 ? (a) : (MIN(a, b)))) +#define MIN_NON_ZERO(a, b) \ + ({ \ + typeof(1 ? (a) : (b)) _a = (a), _b = (b); \ + _a == 0 ? _b : (_b == 0 || _b > _a) ? _a : _b; \ + }) #endif /* Round number down to multiple */ diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c index 42ce1dfcff77..d77add79b218 100644 --- a/accel/tcg/translate-all.c +++ b/accel/tcg/translate-all.c @@ -2565,9 +2565,9 @@ int page_check_range(target_ulong start, target_ulong len, int flags) /* This function should never be called with addresses outside the guest address space. If this assert fires, it probably indicates a missing call to h2g_valid. */ -#if TARGET_ABI_BITS > L1_MAP_ADDR_SPACE_BITS - assert(start < ((target_ulong)1 << L1_MAP_ADDR_SPACE_BITS)); -#endif + if (TARGET_ABI_BITS > L1_MAP_ADDR_SPACE_BITS) { + assert(start < ((target_ulong)1 << L1_MAP_ADDR_SPACE_BITS)); + } if (len == 0) { return 0; diff --git a/migration/qemu-file.c b/migration/qemu-file.c index 1c3a358a140d..be21518c5708 100644 --- a/migration/qemu-file.c +++ b/migration/qemu-file.c @@ -31,7 +31,7 @@ #include "qapi/error.h" #define IO_BUF_SIZE 32768 -#define MAX_IOV_SIZE MIN(IOV_MAX, 64) +#define MAX_IOV_SIZE MIN_CONST(IOV_MAX, 64) struct QEMUFile { const QEMUFileOps *ops;
I'm not aware of any immediate bugs in qemu where a second runtime evalution of the arguments to MIN() or MAX() causes a problem, but proactively preventing such abuse is easier than falling prey to an unintended case down the road. At any rate, here's the conversation that sparked the current patch: https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg05718.html Update the MIN/MAX macros to only evaluate their argument once at runtime; this uses typeof(1 ? (a) : (b)) to ensure that we are promoting the temporaries to the same type as the final comparison (we have to trigger type promotion, as typeof(bitfield) won't compile; and we can't use typeof((a) + (b)) or even typeof((a) + 0), as some of our uses of MAX are on void* pointers where such addition is undefined). However, we are unable to work around gcc refusing to compile ({}) in a constant context (such as the array length of a static variable), even when only used in the dead branch of a __builtin_choose_expr(), so we have to provide a second macro pair MIN_CONST and MAX_CONST for use when both arguments are known to be compile-time constants and where the result must also be usable as a constant; this second form evaluates arguments multiple times but that doesn't matter for constants. By using a void expression as the expansion if a non-constant is presented to this second form, we can enlist the compiler to ensure the double evaluation is not attempted on non-constants. Alas, as both macros now rely on compiler intrinsics, they are no longer usable in preprocessor #if conditions; those will just have to be open-coded or the logic rewritten into #define or runtime 'if' conditions (but where the compiler dead-code-elimination will probably still apply). I tested that both gcc 10.1.1 and clang 10.0.0 produce errors for all forms of macro mis-use. As the errors can sometimes be cryptic, I'm demonstrating the gcc output: Use of MIN when MIN_CONST is needed: In file included from /home/eblake/qemu/qemu-img.c:25: /home/eblake/qemu/include/qemu/osdep.h:249:5: error: braced-group within expression allowed only inside a function 249 | ({ \ | ^ /home/eblake/qemu/qemu-img.c:92:12: note: in expansion of macro ‘MIN’ 92 | char array[MIN(1, 2)] = ""; | ^~~ Use of MIN_CONST when MIN is needed: /home/eblake/qemu/qemu-img.c: In function ‘is_allocated_sectors’: /home/eblake/qemu/qemu-img.c:1225:15: error: void value not ignored as it ought to be 1225 | i = MIN_CONST(i, n); | ^ Use of MIN in the preprocessor: In file included from /home/eblake/qemu/accel/tcg/translate-all.c:20: /home/eblake/qemu/accel/tcg/translate-all.c: In function ‘page_check_range’: /home/eblake/qemu/include/qemu/osdep.h:249:6: error: token "{" is not valid in preprocessor expressions 249 | ({ \ | ^ Fix the resulting callsites that used #if or computed a compile-time constant min or max to use the new macros. cpu-defs.h is interesting, as CPU_TLB_DYN_MAX_BITS is sometimes used as a constant and sometimes dynamic. Signed-off-by: Eric Blake <eblake@redhat.com> --- v2 was: https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00727.html v3: avoid __auto_type [Richard], document other approaches that fail [Dave], rebase to master v4: use ((void)0) instead of __builtin_unreachable [Dave], update comments --- hw/usb/hcd-xhci.h | 2 +- include/block/block.h | 4 +-- include/exec/cpu-all.h | 8 +++--- include/exec/cpu-defs.h | 7 +++++- include/qemu/osdep.h | 51 +++++++++++++++++++++++++++++++++------ accel/tcg/translate-all.c | 6 ++--- migration/qemu-file.c | 2 +- 7 files changed, 59 insertions(+), 21 deletions(-)