[13/14] target/i386: use higher-precision arithmetic to compute CF

Commit Message

Paolo Bonzini Oct. 28, 2024, 3:18 p.m. UTC

If the operands of the arithmetic instruction fit within a half-register,
it's easiest to use a comparison instruction to compute the carry.
`
Signed-off-by: Paolo Bonzini <[e-mail]>
---
 target/i386/tcg/cc_helper_template.h.inc | 37 ++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

Comments

Richard Henderson Oct. 29, 2024, 3:46 p.m. UTC | #1

On 10/28/24 15:18, Paolo Bonzini wrote:
> If the operands of the arithmetic instruction fit within a half-register,
> it's easiest to use a comparison instruction to compute the carry.
> `
> Signed-off-by: Paolo Bonzini<[e-mail]>
> ---
>   target/i386/tcg/cc_helper_template.h.inc | 37 ++++++++++++++++++++++++
>   1 file changed, 37 insertions(+)

Reviewed-by: Richard Henderson <[e-mail]>

r~

Patch

diff --git a/target/i386/tcg/cc_helper_template.h.inc b/target/i386/tcg/cc_helper_template.h.inc
index 8af8b8539f9..f29a6dfb77c 100644
--- a/target/i386/tcg/cc_helper_template.h.inc
+++ b/target/i386/tcg/cc_helper_template.h.inc
@@ -22,12 +22,17 @@ 
 #if DATA_BITS == 8
 #define SUFFIX b
 #define DATA_TYPE uint8_t
+#define WIDER_TYPE uint32_t
 #elif DATA_BITS == 16
 #define SUFFIX w
 #define DATA_TYPE uint16_t
+#define WIDER_TYPE uint32_t
 #elif DATA_BITS == 32
 #define SUFFIX l
 #define DATA_TYPE uint32_t
+#if HOST_LONG_BITS <= 64
+#define WIDER_TYPE uint64_t
+#endif
 #elif DATA_BITS == 64
 #define SUFFIX q
 #define DATA_TYPE uint64_t
@@ -62,9 +67,18 @@  static uint32_t glue(compute_all_adc, SUFFIX)(DATA_TYPE dst, DATA_TYPE src1,
                                          DATA_TYPE src3)
 {
     uint32_t cf, pf, af, zf, sf, of;
+
+#ifdef WIDER_TYPE
+    WIDER_TYPE src13 = (WIDER_TYPE) src1 + (WIDER_TYPE) src3;
+    DATA_TYPE src2 = dst - src13;
+
+    cf = dst < src13;
+#else
     DATA_TYPE src2 = dst - src1 - src3;
 
     cf = (src3 ? dst <= src1 : dst < src1);
+#endif
+
     pf = compute_pf(dst);
     af = (dst ^ src1 ^ src2) & 0x10;
     zf = (dst == 0) << 6;
@@ -76,7 +90,13 @@  static uint32_t glue(compute_all_adc, SUFFIX)(DATA_TYPE dst, DATA_TYPE src1,
 static int glue(compute_c_adc, SUFFIX)(DATA_TYPE dst, DATA_TYPE src1,
                                        DATA_TYPE src3)
 {
+#ifdef WIDER_TYPE
+    WIDER_TYPE src13 = (WIDER_TYPE) src1 + (WIDER_TYPE) src3;
+
+    return dst < src13;
+#else
     return src3 ? dst <= src1 : dst < src1;
+#endif
 }
 
 static uint32_t glue(compute_all_sub, SUFFIX)(DATA_TYPE dst, DATA_TYPE src2)
@@ -104,9 +124,18 @@  static uint32_t glue(compute_all_sbb, SUFFIX)(DATA_TYPE dst, DATA_TYPE src2,
                                          DATA_TYPE src3)
 {
     uint32_t cf, pf, af, zf, sf, of;
+
+#ifdef WIDER_TYPE
+    WIDER_TYPE src23 = (WIDER_TYPE) src2 + (WIDER_TYPE) src3;
+    DATA_TYPE src1 = dst + src23;
+
+    cf = src1 < src23;
+#else
     DATA_TYPE src1 = dst + src2 + src3;
 
     cf = (src3 ? src1 <= src2 : src1 < src2);
+#endif
+
     pf = compute_pf(dst);
     af = (dst ^ src1 ^ src2) & 0x10;
     zf = (dst == 0) << 6;
@@ -118,9 +147,16 @@  static uint32_t glue(compute_all_sbb, SUFFIX)(DATA_TYPE dst, DATA_TYPE src2,
 static int glue(compute_c_sbb, SUFFIX)(DATA_TYPE dst, DATA_TYPE src2,
                                        DATA_TYPE src3)
 {
+#ifdef WIDER_TYPE
+    WIDER_TYPE src23 = (WIDER_TYPE) src2 + (WIDER_TYPE) src3;
+    DATA_TYPE src1 = dst + src23;
+
+    return src1 < src23;
+#else
     DATA_TYPE src1 = dst + src2 + src3;
 
     return (src3 ? src1 <= src2 : src1 < src2);
+#endif
 }
 
 static uint32_t glue(compute_all_logic, SUFFIX)(DATA_TYPE dst, DATA_TYPE src1)
@@ -258,3 +294,4 @@  static int glue(compute_c_blsi, SUFFIX)(DATA_TYPE dst, DATA_TYPE src1)
 #undef DATA_TYPE
 #undef DATA_MASK
 #undef SUFFIX
+#undef WIDER_TYPE