https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95447
Bug ID: 95447
Summary: cmpstrn peepholes are out of date
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: hjl.tools at gmail dot com
CC: crazylht at gmail dot com, hubicka at ucw dot cz, ubizjak
at gmail dot com
Target Milestone: ---
Target: i386,x86-64
cmpstrn peepholes are out of date. i386.md has
;; Peephole optimizations to clean up after cmpstrn*. This should be
;; handled in combine, but it is not currently up to the task.
;; When used for their truth value, the cmpstrn* expanders generate
;; code like this:
;;
;; repz cmpsb
;; seta %al
;; setb %dl
;; cmpb %al, %dl
;; jcc label
;;
;; The intermediate three instructions are unnecessary.
But GCC no longer generates such code sequence. Instead, we generate
[hjl@gnu-cfl-2 pr95151]$ cat s1.i
extern void bar (void);
void
func (char *d, unsigned int l)
{
if (__builtin_strncmp (d, "foo", l))
bar ();
}
[hjl@gnu-cfl-2 pr95151]$ gcc -O2 -minline-all-stringops -S s1.i
[hjl@gnu-cfl-2 pr95151]$ cat s1.s
.file "s1.i"
.text
.section .rodata.str1.1,"aMS",@progbits,1
.LC0:
.string "foo"
.text
.p2align 4
.globl func
.type func, @function
func:
.LFB0:
.cfi_startproc
cmpl $4, %esi
movl %esi, %ecx
movl $4, %eax
movq %rdi, %r8
cmova %rax, %rcx
movl $.LC0, %edi
movq %r8, %rsi
cmpq %rcx, %rcx
repz cmpsb
seta %al
sbbb $0, %al
testb %al, %al
jne .L4
ret
.p2align 4,,10
.p2align 3
.L4:
jmp bar
.cfi_endproc
.LFE0:
.size func, .-func
.ident "GCC: (GNU) 10.1.1 20200507 (Red Hat 10.1.1-1)"
.section .note.GNU-stack,"",@progbits
[hjl@gnu-cfl-2 pr95151]$
The cmpstrn peepholes never kick in.
