https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86525
Bug ID: 86525
Summary: [missed-optimization] extraneous instruction emitted
in switch converted to look-uptable load
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: a...@cloudius-systems.com
Target Milestone: ---
The code
enum xx { x1 = 1, x2 = 2, x3 = 3, x4, x5, x6 };
unsigned char f(xx x) {
switch (x) {
case xx::x1:
return 2;
case xx::x2:
return 2;
case xx::x3:
return 7;
case xx::x4:
return 7;
case xx::x5:
return 7;
case xx::x6:
return 9;
}
}
compiles to (thanks godbolt)
f(xx):
leal -1(%rdi), %eax
movzbl CSWTCH.1(%rax), %eax
ret
CSWTCH.1:
.byte 2
.byte 2
.byte 7
.byte 7
.byte 7
.byte 9
which is lovely, but the lea instruction can be folded into the movzbl
instruction:
movzbl CSWTCH.1 - 1(%rax), %eax
This assumes that CSWTCH.1 is placed at offset != 0.
