https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88841
Bug ID: 88841
Summary: Missed optimization transforming cascading ||s into a
bit select
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: matt at godbolt dot org
Target Milestone: ---
It seems around GCC 7 an optimization was added turning multiple comparisons of
a small range into a bit-select. This optimization seems to be sensitive to the
ordering of the comparisons, which seems like a missed opportunity.
On x86_64 GCCs 7 and above (tested with trunk 9.0 too) with -O2 :
---snip
bool isspc_1(char c)
{
return c == ' '
|| c == '\n'
|| c == '\r'
|| c == '\t';
}
bool isspc_2(char c)
{
return c == ' '
|| c == '\r'
|| c == '\n'
|| c == '\t';
}
--- end snip (see https://godbolt.org/z/ovB_Oz )
...the isspc_2 is optimized using the bit selection optimization, but the
isspc_1 is not. The only difference is the order of the comparisons. It's not
clear to me which is actually faster, but my instinct is the results of these
two functions should be the same code.
