https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100769
Bug ID: 100769
Summary: [D] memcmp() == 0 for small constant strings not
folded
Product: gcc
Version: 10.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: d
Assignee: ibuclaw at gdcproject dot org
Reporter: witold.baryluk+gcc at gmail dot com
Target Milestone: ---
I expect this D code to be quite optimal, but it isn't.
```
extern(C) int memcmp(const void *s1, const void *s2, size_t n);
int recognize3(const char* s) {
return memcmp(s, "stract class", 12) == 0;
}
```
https://godbolt.org/z/vx17WK9rs
It produces a call to memcmp, instead of inlining and specializing the code for
this specific case.
int example.recognize3(const(char*)):
sub rsp, 8
mov edx, 12
mov esi, OFFSET FLAT:.LC0
call memcmp
test eax, eax
sete al
add rsp, 8
movzx eax, al
ret
ldc2 1.24.0 (for D) and clang 11.0.1-2 (for C and C++), and gcc 10.2.1 (for C
and C++) produce close to optimal codes. Similarly ldc2 1.26.0 (for D), and gcc
11.1 (for C and C++):
int example.recognize3(const(char*)):
movabs rcx, 7142836979195081843
xor rcx, qword ptr [rdi]
mov edx, dword ptr [rdi + 8]
xor rdx, 1936941420
xor eax, eax
or rdx, rcx
sete al
ret
and
recognize3:
movabs rax, 7142836979195081843
cmp QWORD PTR [rdi], rax
je .L6
.L2:
mov eax, 1
xor eax, 1
ret
.L6:
xor eax, eax
cmp DWORD PTR [rdi+8], 1936941420
jne .L2
xor eax, 1
ret
Notice, how both gcc, clang and ldc2, compare first 8 bytes of input, then 4
bytes of input. clang and ldc2 just xor/or the result, then return, with no
conditional jumps. gcc does a bit poorer, with more conditionals and more
jumps, but still pretty good and same idea.
gdc however, calls the generic memcmp, that does looping and does about 12
jumps and/or 13 exists.
