https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87205
Bug ID: 87205
Summary: Inefficient code generation for switch
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: pdimov at gmail dot com
Target Milestone: ---
For the following code:
```
void f( int x );
void h( unsigned ix )
{
switch( ix )
{
case 0: f(42); break;
case 1: f(42); break;
case 2: f(42); break;
case 3: f(42); break;
case 4: f(42); break;
case 5: f(42); break;
default: __builtin_unreachable();
}
}
```
g++ 9.0 -O2, -O3 generates:
```
h(unsigned int):
cmp edi, 5
jbe .L5
.L5:
mov edi, 42
jmp f(int)
```
https://godbolt.org/z/4I_Chu
The initial part that compares edi to 5 is redundant.
At -O1 the result is a jump table that doesn't check edi, as expected:
```
h(unsigned int):
sub rsp, 8
mov edi, edi
jmp [QWORD PTR .L4[0+rdi*8]]
```
This is a simplified example; I've stripped the metaprogramming that produces
it. :-)
For comparison, g++ 8.2 produces
```
h(unsigned int):
cmp edi, 5
ja .L2
mov edi, 42
jmp f(int)
h(unsigned int) [clone .cold.0]:
.L2:
```
