https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66822
--- Comment #6 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to H.J. Lu from comment #5)
> X86_TUNE_USE_BT also reduces code size:
>
> [hjl@gnu-tools-1 pr66822]$ cat y.i
> void bar (void);
>
> void
> foo (int x)
> {
> if (x != 2 && x != 3 && x != 10 && x != 11 && x != 17 && x != 18 && x !=
> 23)
> bar ();
> }
> [hjl@gnu-tools-1 pr66822]$ make y.o
> /export/build/gnu/gcc/build-x86_64-linux/gcc/xgcc
> -B/export/build/gnu/gcc/build-x86_64-linux/gcc/ -O2 -m32 -miamcu
> -mtune=iamcu -S -o y.s y.i
> /export/build/gnu/gcc/build-x86_64-linux/gcc/xgcc
> -B/export/build/gnu/gcc/build-x86_64-linux/gcc/ -O2 -m32 -miamcu
> -mtune=iamcu -c -o y.o y.s
> rm y.s
> [hjl@gnu-tools-1 pr66822]$ objdump -dwr y.o
>
> y.o: file format elf32-i386
>
>
> Disassembly of section .text:
>
> 00000000 <foo>:
> 0: 83 f8 17 cmp $0x17,%eax
> 3: 76 0b jbe 10 <foo+0x10>
> 5: e9 fc ff ff ff jmp 6 <foo+0x6> 6: R_386_PC32 bar
> a: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
> 10: ba 0c 0c 86 00 mov $0x860c0c,%edx
> 15: 88 c1 mov %al,%cl
> 17: d3 ea shr %cl,%edx
> 19: 83 e2 01 and $0x1,%edx
> 1c: 74 e7 je 5 <foo+0x5>
> 1e: c3 ret
> [hjl@gnu-tools-1 pr66822]$ /export/build/gnu/gcc/build-x86_64-linux/gcc/xgcc
> -B/export/build/gnu/gcc/build-x86_64-linux/gcc/ -O2 -m32 -miamcu
> -mtune=iamcu -c y.i -mtune-ctrl=use_bt
> [hjl@gnu-tools-1 pr66822]$ objdump -dwr y.o
>
> y.o: file format elf32-i386
>
>
> Disassembly of section .text:
>
> 00000000 <foo>:
> 0: 83 f8 17 cmp $0x17,%eax
> 3: 76 0b jbe 10 <foo+0x10>
> 5: e9 fc ff ff ff jmp 6 <foo+0x6> 6: R_386_PC32 bar
> a: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
> 10: ba 0c 0c 86 00 mov $0x860c0c,%edx
> 15: 0f a3 c2 bt %eax,%edx
> 18: 73 eb jae 5 <foo+0x5>
> 1a: c3 ret
> [hjl@gnu-tools-1 pr66822]$
We can reduce size further by
diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index bca1fee..4f08d0a 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -2556,7 +2556,7 @@ static const struct ptt
processor_target_table[PROCESSOR_max] =
{"i386", &i386_cost, 4, 3, 4, 3, 4},
{"i486", &i486_cost, 16, 15, 16, 15, 16},
{"pentium", &pentium_cost, 16, 7, 16, 7, 16},
- {"iamcu", &iamcu_cost, 16, 7, 16, 7, 16},
+ {"iamcu", &iamcu_cost, 0, 0, 0, 0, 0},
{"pentiumpro", &pentiumpro_cost, 16, 15, 16, 10, 16},
{"pentium4", &pentium4_cost, 0, 0, 0, 0, 0},
{"nocona", &nocona_cost, 0, 0, 0, 0, 0},
and get
00000000 <foo>:
0: 83 f8 17 cmp $0x17,%eax
3: 76 05 jbe a <foo+0xa>
5: e9 fc ff ff ff jmp 6 <foo+0x6> 6: R_386_PC32 bar
a: ba 0c 0c 86 00 mov $0x860c0c,%edx
f: 0f a3 c2 bt %eax,%edx
12: 73 f1 jae 5 <foo+0x5>
14: c3 ret
[hjl@gnu-tools-1 pr66822]$
