gcc 4.8.2 fails to do optimization on global register variables when
compiling on x86_64 Linux.
Consider the following code:
---------------------------------------------------------------------
include <stdint.h>
register uint64_t i0_BP __asm__ ("r14");
register uint64_t i0_SP __asm__ ("r15");
void test(void) {
*((uint64_t*) (i0_SP - 8)) = i0_BP;
i0_BP = i0_SP - 0x8;
i0_SP -= 0x100;
i0_SP = i0_BP;
i0_BP = *((uint64_t*) i0_SP);
i0_SP += 0x8;
return;
}
---------------------------------------------------------------------
Apply either -O3 or -Os option to gcc, the final object file gives the
same results as follows:
---------------------------------------------------------------------
<test>:
0: lea 0xfffffffffffffff8(%r15),%rcx
4: mov %r14,%rdx
7: mov %r15,%rax
a: mov %r14,0xfffffffffffffff8(%r15)
e: mov %rcx,%r14
11: mov %rcx,%r15
14: mov %rdx,%r14
17: mov %rax,%r15
1a: retq
---------------------------------------------------------------------
Here we just try to emulate a function call. In the object file, there are
apparently lots of redundant movs between registers. It seems to be a bug
in gcc since we have already apply the maximum optimization level possible.
Environment:
On CentOS 5.10 (Linux 2.6.18 x86_64) using GCC 4.8.2
Using built-in specs.
COLLECT_GCC=gcc4
COLLECT_LTO_WRAPPER=/usr/local/GNU/gcc-4.8.2/libexec/gcc/x86_64-unknown-linu
x-gnu/4.8.2/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-4.8.2/configure --prefix=/usr/local/GNU/gcc-4.8.2
--enable-clocale=generic
Thread model: posix
gcc version 4.8.2 (GCC)
