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:
PR tree-optimization/95424
* match.pd: Simplify 1 / X where X is an integer.
&& TYPE_UNSIGNED (type))
(trunc_divmod @0 @1))))
+ /* 1 / X -> X == 1 for unsigned integer X.
+ 1 / X -> X >= -1 && X <= 1 ? X : 0 for signed integer X.
+ But not for 1 / 0 so that we can get proper warnings and errors,
+ and not for 1-bit integers as they are edge cases better handled elsewhere. */
+(simplify
+ (trunc_div integer_onep@0 @1)
+ (if (INTEGRAL_TYPE_P (type) && !integer_zerop (@1) && TYPE_PRECISION (type) > 1)
+ (if (TYPE_UNSIGNED (type))
+ (eq @1 { build_one_cst (type); })
+ (with { tree utype = unsigned_type_for (type); }
+ (cond (le (plus (convert:utype @1) { build_one_cst (utype); }) { build_int_cst (utype, 2); })
+ @1 { build_zero_cst (type); })))))
+
/* Combine two successive divisions. Note that combining ceil_div
and floor_div is trickier and combining round_div even more so. */
(for div (trunc_div exact_div)
--- /dev/null
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-optimized" } */
+
+unsigned int f(unsigned int x) {
+ return 1 / x;
+}
+
+/* { dg-final { scan-tree-dump-not "1 / x_..D.;" "optimized" } } */
+/* { dg-final { scan-tree-dump "x_..D. == 1;" "optimized" } } */
--- /dev/null
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-optimized" } */
+
+int f(int x) {
+ return 1 / x;
+}
+
+/* { dg-final { scan-tree-dump-not "1 / x_..D.;" "optimized" } } */
+/* { dg-final { scan-tree-dump ".. <= 2 ? x_..D. : 0;" "optimized" } } */