The follwing program:
#include <stdio.h>
typedef unsigned char(*Calculable)(void);
unsigned char one() { return 1; }
unsigned char two() { return 2; }
static void print(Calculable calculate)
{
printf("%d\n", calculate());
printf("+1: %d\n", calculate() + 1);
}
int main()
{
print(one);
print(two);
return 0;
}
when compiled with GCC 4.5.0 20091211 with -O3 -fwhole-program, outputs the
following relevant chunk of assembly:
00000000004002e0 <_Z3onev>:
4002e0: b8 01 00 00 00 mov eax,0x1
4002e5: c3 ret
4002e6: 66 2e 0f 1f 84 00 00 nop WORD PTR cs:[rax+rax*1+0x0]
4002ed: 00 00 00
00000000004002f0 <_Z3twov>:
4002f0: b8 02 00 00 00 mov eax,0x2
4002f5: c3 ret
4002f6: 66 2e 0f 1f 84 00 00 nop WORD PTR cs:[rax+rax*1+0x0]
4002fd: 00 00 00
0000000000400300 <main>:
400300: 48 83 ec 08 sub rsp,0x8
400304: be 01 00 00 00 mov esi,0x1
400309: bf 34 04 40 00 mov edi,0x400434
40030e: 31 c0 xor eax,eax
400310: e8 73 02 00 00 call 400588 <pri...@plt>
400315: be 02 00 00 00 mov esi,0x2
40031a: bf 2c 04 40 00 mov edi,0x40042c
40031f: 31 c0 xor eax,eax
400321: e8 62 02 00 00 call 400588 <pri...@plt>
400326: be 02 00 00 00 mov esi,0x2
40032b: bf 34 04 40 00 mov edi,0x400434
400330: 31 c0 xor eax,eax
400332: e8 51 02 00 00 call 400588 <pri...@plt>
400337: be 03 00 00 00 mov esi,0x3
40033c: bf 2c 04 40 00 mov edi,0x40042c
400341: 31 c0 xor eax,eax
400343: e8 40 02 00 00 call 400588 <pri...@plt>
400348: 31 c0 xor eax,eax
40034a: 48 83 c4 08 add rsp,0x8
40034e: c3 ret
40034f: 90 nop
GCC correctly folds the functions one() and two(), and does all the
compile-time math (yay!). however, the one() and two() functions are still
emitted to the binary even though they are now dead. There is a concern that
this negatively affects code locality on cache-limited platforms where a
profile-guided optimization is non-trivial. We are currently seeing some cache
misses and this example embodies one of the roadblocks to eliminating them
while keeping the code well-encapsulated.
--
Summary: dead code not eliminated during folding with whole-
program
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: matt at use dot net
GCC build triplet: x86_64-unknown-linux-gnu
GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42371
