https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127420
--- Comment #8 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Haochen Jiang from comment #7)
> > What are typical cases? Are they C++ programs?
>
> All SPEC2026 cases are CXX + C or CXX. For SPEC2017, 511 is CXX + C, the
> remaining for mentioned is C only.
>
> For 735, the case is a 20-case switch in a loop:
>
> c3c090: inc %rdx
> c3c093: mov %ecx,(%rax)
> c3c095: movq $0x0,0x8(%rax)
> c3c09d: cmp %rsi,%rdx
> c3c0a0: jae c3c118
> c3c0a2: cmp $0x13,%edx
> c3c0a5: ja c3c2ff
> c3c0ab: mov %edx,%ecx
> c3c0ad: jmp *0x1021900(,%rcx,8) # 20-entry tablejump <- 7
> bytes
> c3c0b4: nopl 0x0(%rax)
> c3c0b8: xor %ecx,%ecx # <-- 932,626 hits land here
> c3c0ba: add $0x10,%rax
> c3c0be: jmp c3c090
>
> The 84% c3c0ad indirect jmp FALLTHRU to c3c0b8, leading to total 26.7% for
> 735.
My patch only applies to sibcall, not table jump:
[hjl@gnu-zen4-1 pr127420]$ cat t.c
extern void bar0 (void);
extern void bar1 (void);
extern void bar2 (void);
extern void bar3 (void);
extern void bar4 (void);
void
foo (int i)
{
switch (i)
{
case 0: bar0 (); break;
case 1: bar1 (); break;
case 2: bar2 (); break;
case 3: bar3 (); break;
case 4: bar4 (); break;
}
}
[hjl@gnu-zen4-1 pr127420]$ make
/export/build/gnu/tools-build/gcc-gitlab-test-debug/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/tools-build/gcc-gitlab-test-debug/build-x86_64-linux/gcc/
-O2 -fno-plt -S t.c
[hjl@gnu-zen4-1 pr127420]$ cat t.s
.file "t.c"
.text
.p2align 4
.globl foo
.type foo, @function
foo:
.LFB0:
.cfi_startproc
cmpl $4, %edi
ja .L1
movl %edi, %edi
jmp *.L4(,%rdi,8)
.section .rodata
.align 8
.align 4
.L4:
.quad .L8
.quad .L7
.quad .L6
.quad .L5
.quad .L3
.text
.p2align 4,,10
.p2align 3
.L5:
jmp *bar3@GOTPCREL(%rip)
ud2
.p2align 4,,10
.p2align 3
.L3:
jmp *bar4@GOTPCREL(%rip)
ud2
.p2align 4,,10
.p2align 3
.L8:
jmp *bar0@GOTPCREL(%rip)
ud2
.p2align 4,,10
.p2align 3
.L7:
jmp *bar1@GOTPCREL(%rip)
ud2
.p2align 4,,10
.p2align 3
.L6:
jmp *bar2@GOTPCREL(%rip)
ud2
.L1:
ret
.cfi_endproc
.LFE0:
.size foo, .-foo
.ident "GCC: (GNU) 17.0.0 20260918 (experimental)"
.section .note.GNU-stack,"",@progbits
[hjl@gnu-zen4-1 pr127420]$
Can you measure its performance impact?
