https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92985
Bug ID: 92985
Summary: missed optimization opportunity for switch linear
transformation
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
From Peter Dimov on Slack:
#include <cstddef>
struct X
{
int x, y, z;
int operator[]( std::size_t i ) const noexcept
{
switch( i )
{
case 0: return x;
case 1: return y;
case 2: return z;
default: __builtin_unreachable();
}
}
};
int f( X const& x, std::size_t i )
{
return x[ i ];
}
compiled with -O2 (or -O3) emits:
f(X const&, unsigned long):
cmp rsi, 1
je .L2
cmp rsi, 2
jne .L6
mov eax, DWORD PTR [rdi+8]
ret
.L2:
mov eax, DWORD PTR [rdi+4]
ret
.L6:
mov eax, DWORD PTR [rdi]
ret
But it should be able to just emit:
mov eax, DWORD PTR [rdi+rsi*4]
ret
