On 1/19/2022 11:42 AM, Zhao Wei Liew via Gcc-patches wrote:
This patch implements an optimization for the following C++ code:
int f(int x) {
return 1 / x;
}
int f(unsigned int x) {
return 1 / x;
}
Before this patch, x86-64 gcc -std=c++20 -O3 produces the following assembly:
f(int):
xor edx, edx
mov eax, 1
idiv edi
ret
f(unsigned int):
xor edx, edx
mov eax, 1
div edi
ret
In comparison, clang++ -std=c++20 -O3 produces the following assembly:
f(int):
lea ecx, [rdi + 1]
xor eax, eax
cmp ecx, 3
cmovb eax, edi
ret
f(unsigned int):
xor eax, eax
cmp edi, 1
sete al
ret
Clang's output is more efficient as it avoids expensive div operations.
With this patch, GCC now produces the following assembly:
f(int):
lea eax, [rdi + 1]
cmp eax, 2
mov eax, 0
cmovbe eax, edi
ret
f(unsigned int):
xor eax, eax
cmp edi, 1
sete al
ret
which is virtually identical to Clang's assembly output. Any slight differences
in the output for f(int) is possibly related to a different missed optimization.
v2: https://gcc.gnu.org/pipermail/gcc-patches/2022-January/587751.html
Changes from v2:
1. Refactor from using a switch statement to using the built-in
if-else statement.
v1: https://gcc.gnu.org/pipermail/gcc-patches/2022-January/587634.html
Changes from v1:
1. Refactor common if conditions.
2. Use build_[minus_]one_cst (type) to get -1/1 of the correct type.
3. Match only for TRUNC_DIV_EXPR and TYPE_PRECISION (type) > 1.
gcc/ChangeLog:
* match.pd: Simplify 1 / X where X is an integer.
gcc/testsuite/ChangeLog:
* gcc.dg/tree-ssa/divide-6.c: New test.
* gcc.dg/tree-ssa/divide-7.c: New test.
Thanks. Given the original submission and most of the review work was
done prior to stage3 closing, I went ahead and installed this on the trunk.
jeff
