https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105137
Bug ID: 105137
Summary: Missed optimization 64-bit adds and shifts
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: andre.schackier at gmail dot com
Target Milestone: ---
Given the following source code [godbolt](https://godbolt.org/z/8KMMhefqY)
#include <stdint.h>
typedef __int128_t int128_t;
int64_t foo(int128_t a, int64_t b, int cond) {
if (cond) {
a += ((int128_t)b) << 64;
}
return a >> 64;
}
int64_t bar(int128_t a, int64_t b, int cond) {
int64_t r = a >> 64;
if (cond) {
r += b;
}
return r;
}
Compiling with "-O3" we get:
foo:
mov rax, rsi
mov rsi, rdi
mov rdi, rax
test ecx, ecx
je .L2
xor r8d, r8d
add rsi, r8
adc rdi, rdx
.L2:
mov rax, rdi
ret
bar:
add rdx, rsi
mov rax, rsi
test ecx, ecx
cmovne rax, rdx
ret
Although both functions do the same, gcc implements worse code for foo.
Credits: This was entirely found by Trevor Spiteri reported at the llvm-project
here: https://github.com/llvm/llvm-project/issues/54718
