https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105126
Bug ID: 105126
Summary: Optimization regression gcc inserts not needed movsx
when using switch statement
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: regression
Assignee: unassigned at gcc dot gnu.org
Reporter: andre.schackier at gmail dot com
Target Milestone: ---
Given the following source code:
```cpp
bool is_bin_0(const char c) { return (c == '0' || c == '1'); }
bool is_bin_1(const char c) {
switch (c) {
case '0':
case '1':
return true;
default:
return false;
}
}
```
compiling with `-O3` gives the following output:
```asm
is_bin_0(char):
sub edi, 48
cmp dil, 1
setbe al
ret
is_bin_1(char):
movsx edi, dil
sub edi, 48
cmp edi, 1
setbe al
ret
```
The version using a switch generates an extra movsx instruction which, as far
is I understand the x86 CPU registers is not needed since DIL is the lower 8
bits of EDI anyways, but correct me if I'm wrong there.
This also seems to be a regression introduced with GCC-8.1 since GCC-4.5 to
GCC-7.5 generate the same assembly as for the first and second function and all
version after that including trunk produce different outputs.
[link to godbolt](https://godbolt.org/z/r3vhYbK6o)
