https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95886
Bug ID: 95886
Summary: suboptimal memcpy with embedded zero bytes
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
While testing the fix for pr95189 I noticed that the memcpy expansion into
copy-by-pieces is less than optimal for sequences containing embedded null
bytes. For example, in the test case below, the memcpy call in f() is expanded
into what looks like a more efficient sequence than the equivalent memcpy call
in g(). The only difference between the two is that the former copies a
sequence of non-zero bytes while among the bytes copied by the latter is a null
byte. Clang emits the same code for g() as GCC does for f().
$ cat z.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout -o/dev/stdout
z.c
const char a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
const char b[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8 };
void f (void *d)
{
__builtin_memcpy (d, a, 9); // optimal
}
void g (void *d)
{
__builtin_memcpy (d, b, 9); // suboptimal
}
.file "z.c"
.text
;; Function f (f, funcdef_no=0, decl_uid=1932, cgraph_uid=1, symbol_order=2)
f (void * d)
{
<bb 2> [local count: 1073741824]:
__builtin_memcpy (d_2(D), &a, 9); [tail call]
return;
}
.p2align 4
.globl f
.type f, @function
f:
.LFB0:
.cfi_startproc
movabsq $578437695752307201, %rax
movb $9, 8(%rdi)
movq %rax, (%rdi)
ret
.cfi_endproc
.LFE0:
.size f, .-f
;; Function g (g, funcdef_no=1, decl_uid=1935, cgraph_uid=2, symbol_order=3)
g (void * d)
{
<bb 2> [local count: 1073741824]:
__builtin_memcpy (d_2(D), &b, 9); [tail call]
return;
}
.p2align 4
.globl g
.type g, @function
g:
.LFB1:
.cfi_startproc
movq b(%rip), %rax
movq %rax, (%rdi)
movzbl b+8(%rip), %eax
movb %al, 8(%rdi)
ret
.cfi_endproc
.LFE1:
.size g, .-g
.globl b
.section .rodata
.align 8
.type b, @object
.size b, 10
b:
.string ""
.string "\001\002\003\004\005\006\007\b"
.globl a
.align 8
.type a, @object
.size a, 10
a:
.string "\001\002\003\004\005\006\007\b\t"
.ident "GCC: (GNU) 10.1.1 20200527"
.section .note.GNU-stack,"",@progbits
