https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68000
Bug ID: 68000
Summary: Suboptimal ternary operator codegen
Product: gcc
Version: 5.1.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: thaines.astro at gmail dot com
Target Milestone: ---
Created attachment 36534
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36534&action=edit
Preprocessed C code for "Suboptimal ternary operator codegen"
I think this problem is likely related to bug 23286, bug 56096, bug 64524, and
possibly bug 67879.
struct pair8 { uint8_t x,y; };
uint8_t foo_manual_hoist(struct pair8 *p) {
const uint8_t a = p->x + 1;
return a == p->y ? 0 : a;
}
uint8_t foo(struct pair8 *p) {
return p->x + 1 == p->y ? 0 : p->x + 1;
}
uint8_t foo_if(struct pair8 *p) {
uint8_t a = p->x + 1;
if (a == p->y)
a = 0;
return a;
}
int main() {}
Under gcc-5.1, gcc -m64 -march=native -O3 -g -S -masm=intel -o test.asm test.c
produces
foo_manual_hoist:
movzx eax, BYTE PTR [rdi+1]
mov edx, 0
add eax, 1
cmp al, BYTE PTR [rdi+2]
cmove eax, edx
ret
foo:
movzx edx, BYTE PTR [rdi+1]
movzx ecx, BYTE PTR [rdi+2]
mov eax, edx // weird!
add edx, 1
add eax, 1
cmp edx, ecx
mov edx, 0
cmove eax, edx
ret
foo_if:
movzx eax, BYTE PTR [rdi+1]
mov edx, 0
add eax, 1
cmp al, BYTE PTR [rdi+2]
cmove eax, edx
ret
As expect, the ternary version with the manual hoist of the common
subexpression produces identical codegen as the if version with the same hoist.
What is unexpected, is the duplication of the common subexpression for the
"bare" ternary version (line 3 of the disassembly of foo). It is clear that the
compiler sees the expressions to be the same, but treats them separately
anyway. As an aside, clang-3.6 produces identical code for all three versions.
