https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93539
Bug ID: 93539
Summary: memmove over self with result of string function not
eliminated
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
While experimenting with the test case from pr3538 I noticed that GCC doesn't
take full advantage of the fact that functions like strcpy return their first
argument. In the test case below only the memmove call in the first function
is eliminated. The one in the second one is not (and never has been). Clang
eliminates both as expected (and always has).
$ cat a.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout -o/dev/stdout
a.c
struct A { char a[10]; };
void f (struct A *p)
{
__builtin_strcpy (p->a, "abcd");
char *q = p->a;
__builtin_memmove (q, p->a, sizeof p->a); // eliminated (good)
}
void g (struct A *p)
{
char *q = __builtin_strcpy (p->a, "abcd");
__builtin_memmove (q, p->a, sizeof p->a); // not eliminated
}
.file "a.c"
.text
;; Function f (f, funcdef_no=0, decl_uid=1932, cgraph_uid=1, symbol_order=0)
f (struct A * p)
{
char[10] * _1;
<bb 2> [local count: 1073741824]:
_1 = &p_2(D)->a;
__builtin_memcpy (_1, "abcd", 5); [tail call]
return;
}
.p2align 4
.globl f
.type f, @function
f:
.LFB0:
.cfi_startproc
movl $1684234849, (%rdi)
movb $0, 4(%rdi)
ret
.cfi_endproc
.LFE0:
.size f, .-f
;; Function g (g, funcdef_no=1, decl_uid=1936, cgraph_uid=2, symbol_order=1)
g (struct A * p)
{
char * q;
char[10] * _1;
<bb 2> [local count: 1073741824]:
_1 = &p_2(D)->a;
q_5 = __builtin_memcpy (_1, "abcd", 5);
__builtin_memmove (q_5, _1, 10); [tail call]
return;
}
.p2align 4
.globl g
.type g, @function
g:
.LFB1:
.cfi_startproc
movl $1684234849, (%rdi)
movl $10, %edx
movq %rdi, %rsi
movb $0, 4(%rdi)
jmp memmove
.cfi_endproc
.LFE1:
.size g, .-g
.ident "GCC: (GNU) 10.0.1 20200131 (experimental)"
.section .note.GNU-stack,"",@progbits
