http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56727
Bug #: 56727
Summary: [4.7/4.8] [missed-optimization] Recursive call goes
through the PLT unnecessarily
Classification: Unclassified
Product: gcc
Version: 4.7.2
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c
AssignedTo: [email protected]
ReportedBy: [email protected]
Consider the following code, compiled with -O2 -fPIC either in C or in C++:
===
void f(TYPE b)
{
f(0);
}
===
if TYPE is a type of 32- or 64-bit width (int, unsigned, long, long long), GCC
generates the following code (-m32, -mx32 or -m64):
===
f:
.L2:
jmp .L2
===
If TYPE is shorter than 32-bit (bool, _Bool, char, short), GCC generates the
following code (-mx32, -m64):
===
f:
xorl %edi, %edi
jmp f@PLT
===
and much worse code for -m32.
For whatever reason, GCC decided to place the call via the PLT. That's a the
missed optimisation: if this function was called, then the PLT must resolve
back to itself. What's more, since the argument wasn't used, it's also
unnecessary to set it.
The output happens without -fPIC, but in that case there is no PLT.
Tested on:
GCC 4.7.2 (as shipped by Fedora 17)
GCC 4.9 (trunk build from 20130318)
This is a contrived example (infinite recursion) that no one would write in
their sane mind. But it might point to missed optimisations in legitimate
recursive functions.
