https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111378
Bug ID: 111378
Summary: Missed optimization for comparing with exact_log2
constants
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: lis8215 at gmail dot com
Target Milestone: ---
The simple example below produces suboptimal code on many targets where
exact_log2 constant can't be represented as immediate operand.
(confirmed MIPS/PPC64/SPARC/RISC-V)
extern void do_something(char* p);
extern void do_something_other(char* p);
void test(char* p, uint32_t ch)
{
if (ch < 0x10000)
{
do_something(p);
}
else /* ch >= 0x10000 */
{
do_something_other(p);
}
}
However, instead of direct comparing with constant we can use shift & compare
to zero:
e.g. (ch < 0x10000) can be transformed into ((ch >> 16) == 0) which is usually
shorter & faster on many targets.
The condition appears in real world rarely AFAIK - 20-30 occurences per million
asm instructions. Fun fact: many of them related to unicode transformations.
