https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116013
Bug ID: 116013
Summary: Missed optimization opportunity with andn involving
consts
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: bisqwit at iki dot fi
Target Milestone: ---
Below are two short functions which work identically. While GCC utilizes the
ANDN instruction (of Intel BMI1) for test2, it fails to see that it could do
the same with test1.
#include <stdint.h>
uint64_t test1(uint64_t value)
{
return ~(value | 0x7F7F7F7F7F7F7F7F);
}
uint64_t test2(uint64_t value)
{
return ~value & ~0x7F7F7F7F7F7F7F7F;
}
Assembler listings of both functions are below (-Ofast -mbmi):
test1:
movabsq $9187201950435737471, %rdx
movq %rdi, %rax
orq %rdx, %rax
notq %rax
ret
test2:
movabsq $-9187201950435737472, %rax
andn %rax, %rdi, %rax
ret
Tested compiler version:
GCC: (Debian 14-20240330-1) 14.0.1 20240330 (experimental) [master
r14-9728-g6fc84f680d0]
This optimization makes only sense if one of the operands is a compile-time
constant. If neither operand is a compile-time constant, then the opposite
optimization makes more sense — which GCC already does.
It is also worth noting, that GCC already compiles ~(var1 | ~var2) into ~var1 &
var2, utilizing ANDN. This is good.
