https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101050
Bug ID: 101050
Summary: Range comparisons with more significant bits constant
are not optimised into masked equality tests
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: felix.von.s at posteo dot de
Target Milestone: ---
Target: x86_64-linux-gnu
Created attachment 50995
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50995&action=edit
Sample functions f and g
Attached sample functions f and g have identical behaviour, but GCC optimises
them differently at -O3. The former is optimised into an arithmetic offset and
unsigned comparison, while the latter compiles into bit masking and an equality
test, as written. I presume it would be advantageous to reduce the former to
the latter whenever possible (that is, to simplify (C <= x && x < C + (1 << N))
where (C & ((1 << N) - 1)) == 0 into (x & ~(1 << N)) == C), since the
operations involved are slightly weaker. The Other Compiler does this.
And yes, the values in the example are not accidental. This comes up in Unicode
processing, so I think it’s worth the trouble.
