https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102927
Bug ID: 102927
Summary: Failure to optimize series of if-else to use array
when possible
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
int foo(int i) {
if (i == 0)
return 52;
else if (i == 1)
return 77;
else if (i == 2)
return 91;
else if (i == 3)
return 10;
else
return 42;
}
int bar(int i) {
switch (i) {
case 0:
return 52;
case 1:
return 77;
case 2:
return 91;
case 3:
return 10;
default:
return 42;
}
}
int baz(int i)
{
static const int results[] = {52, 77, 91, 10};
if (__builtin_expect_with_probability((unsigned)i < 4, 1, 0.5))
return results[(unsigned)i];
return 42;
}
foo can be optimized to be equivalent to baz (like bar is). This optimization
is done by LLVM, but not by GCC.
PS: I've observed that making the if-else chain longer triggers the
optimization. Is GCC considering the if-else chain to be faster than an array
access ? Because in that case, it seems like bar should be optimized to an
if-else chain (perhaps along with bar).
