| Message ID | 200907100928.07369.elendil@planet.nl (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Friday 10 July 2009, Frans Pop wrote: > On Thu, 9 Apr 2009, Linus Torvalds wrote: > > On Thu, 9 Apr 2009, Andrew Morton wrote: > > > -fwrapv killed Barry's gcc-4.1.2-compiled kernel in 2.6.27.x, > > > 2.6.28.x and presumably 2.6.29, 2.6.30. > > > > Auughh. I hate compiler bugs. They're horrible to debug. > > > > I _think_ 'fwrapv' only really matters with gcc-4.3, so maybe we > > could just enable it for new versions. > > > > HOWEVER, I also wonder if we could instead of "-fwrapv" use > > "-fno-strict-overflow". They are apparently subtly different, and > > maybe the bug literally only happens with -fwrapv. > > > > Barry, can you see if that simple "replace -fwrapv with > > -fno-strict-overflow" works for you? Prompted by the same suggestion from Ben Hutchings I checked this too, but -fno-strict-overflow was only introduced in gcc 4.2. So using it instead of -fwrapv *would* fix the problem for gcc 4.1, but *only* because it would effectively do the same as the patch I proposed: not add an option at all for gcc 4.1. So that change seems illogical unless there are other reasons to prefer -fno-strict-overflow over -fwrapv (well, it would avoid the gcc version check). It does however make it somewhat more logical to change the test in my proposed patch to also allow -fwrapv for gcc 4.2. Cheers, FJP -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Fri, 10 Jul 2009, Frans Pop wrote: > > Prompted by the same suggestion from Ben Hutchings I checked this too, > but -fno-strict-overflow was only introduced in gcc 4.2. > So using it instead of -fwrapv *would* fix the problem for gcc 4.1, but > *only* because it would effectively do the same as the patch I proposed: > not add an option at all for gcc 4.1. > > So that change seems illogical unless there are other reasons to > prefer -fno-strict-overflow over -fwrapv (well, it would avoid the > gcc version check). > > It does however make it somewhat more logical to change the test in my > proposed patch to also allow -fwrapv for gcc 4.2. Hmm. It all really makes me suspect that we should really be using -fno-strict-overflow instead. That not only apparently avoids the unnecessary gcc version check (by virtue of the option only existing in compilers that don't have the problem), but qutie frankly, one of the core people involved with the whole thing (Ian Lance Taylor) seems to think it's the better option. See for example http://www.airs.com/blog/archives/120 on how gcc actually generates better code with -fno-strict-overflow. I added Ian to the cc. Ian: we generally do try to be careful and use explicit unsigned types for code that cares about overflow, but we use -fwrapv because there have been some cases where we didn't (and used pointer comparisons or signed integers). The problem is that apparently gcc-4.1.x was literally generating buggy code with -fwrapv. So now the choice for us is between switching to an explicit version test: -KBUILD_CFLAGS += $(call cc-option,-fwrapv) +KBUILD_CFLAGS += $(shell if [ $(call cc-version) -ge 0402 ]; then \ + echo $(call cc-option,-fwrapv); fi ;) or just switching to -fno-strict-overflow instead: -KBUILD_CFLAGS += $(call cc-option,-fwrapv) +KBUILD_CFLAGS += $(call cc-option,-fno-strict-overflow) which avoids the buggy gcc versions because it's simply not even supported by gcc-4.1.x (and even if that wasn't the case, possibly because only 'wrapv' is the problematic case - apparently the difference _does_ matter to gcc). From everything I have been able to find, I really prefer the second version. Not only is the patch cleaner, but it looks like code generation is better too (for some inexplicable reason, but I suspect it's because -fno-strict-overflow is just saner). Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Linus Torvalds <torvalds@linux-foundation.org> writes: > Ian: we generally do try to be careful and use explicit unsigned types for > code that cares about overflow, but we use -fwrapv because there have been > some cases where we didn't (and used pointer comparisons or signed > integers). > > The problem is that apparently gcc-4.1.x was literally generating buggy > code with -fwrapv. So now the choice for us is between switching to an > explicit version test: > > -KBUILD_CFLAGS += $(call cc-option,-fwrapv) > +KBUILD_CFLAGS += $(shell if [ $(call cc-version) -ge 0402 ]; then \ > + echo $(call cc-option,-fwrapv); fi ;) > > or just switching to -fno-strict-overflow instead: > > -KBUILD_CFLAGS += $(call cc-option,-fwrapv) > +KBUILD_CFLAGS += $(call cc-option,-fno-strict-overflow) > > which avoids the buggy gcc versions because it's simply not even supported > by gcc-4.1.x (and even if that wasn't the case, possibly because only > 'wrapv' is the problematic case - apparently the difference _does_ > matter to gcc). My instinctive advice is that y'all should track down and fix the cases where the program relies on signed overflow being defined. However, if that is difficult--and it is--then I agree that -fno-strict-overflow is preferable when using a compiler which supports it (gcc 4.2.0 and later). (The gcc 4.2 and later option -Wstrict-overflow=N can help find the cases where a program relies on defined signed overflow, but only if somebody is patient enough to wade through all the false positives.) Ian -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/Makefile b/Makefile index 0aeec59..2f8756e 100644 --- a/Makefile +++ b/Makefile @@ -565,7 +565,8 @@ KBUILD_CFLAGS += $(call cc-option,-Wdeclaration-after-statement,) KBUILD_CFLAGS += $(call cc-option,-Wno-pointer-sign,) # disable invalid "can't wrap" optimizations for signed / pointers -KBUILD_CFLAGS += $(call cc-option,-fwrapv) +KBUILD_CFLAGS += $(shell if [ $(call cc-version) -ge 0430 ]; then \ + echo $(call cc-option,-fwrapv); fi ;) # revert to pre-gcc-4.4 behaviour of .eh_frame