https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91680
Bug ID: 91680
Summary: Integer promotion quirk prevents efficient power of 2
division
Product: gcc
Version: 9.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: kim.nilsson at effnet dot com
Target Milestone: ---
Created attachment 46841
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46841&action=edit
Preprocessed source
For the function,
uint_fast32_t foo_a(uint_fast8_t x) {
const uint_fast32_t q = 1 << x;
return 256 / q;
}
GCC-{8.*,9.*} generates inefficient division instructions for both x86_64 and
ARM. Since 'q' is a power of 2, the result should be a single right shift.
Indeed, if the function is changed to,
uint_fast32_t foo_b(uint_fast8_t x) {
return 256 / (1 << x); // This is, of course, equivalent to (256 >> x)
},
the expected instructions are generated. Counter-intuitively, if the original
function is instead changed slightly to,
uint_fast32_t foo_c(uint_fast8_t x) {
const uint_fast32_t q = (uint_fast32_t)1 << x;
return 256 / q;
},
the expected instructions are once again generated.
See https://godbolt.org/z/37K0Ig for examples.
