https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85516
Bug ID: 85516
Summary: [missed-optimization] gcc does not convert multiple
compares against constants to a shift+bitmask test
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: a...@cloudius-systems.com
Target Milestone: ---
Consider
=== example starts ===
enum class E { a, b, c, d, e, f, g, h, i, j };
bool check_mask(E v) {
return v == E::b || v == E::e || v == E::f || v == E::j;
}
=== example ends ===
This can be compiled to just three instructions (x86):
mov $0x232, %eax
bt %edi, %eax
setc %al
but instead gcc compiles it to:
cmpl $1, %edi
sete %al
cmpl $4, %edi
sete %dl
orb %dl, %al
jne .L1
subl $5, %edi
andl $-5, %edi
sete %al
.L1:
which is three times as large and contains a possibly unpredictable branch.
More bits in the mask will presumably generate larger code.
