this may or may not be the same as #29775.
% cat extra-mov.c <<EOF
unsigned *table;
unsigned shift;
unsigned foo1(unsigned p)
{
unsigned block = p >> shift;
unsigned value = table[block];
return (value >> 8);
}
EOF
% gcc -g -O3 -Wall -c -o extra-mov.o extra-mov.c
% objdump -dr extra-mov.o
extra-mov.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <foo1>:
0: 8b 0d 00 00 00 00 mov 0(%rip),%ecx # 6 <foo1+0x6>
2: R_X86_64_PC32 shift+0xfffffffffffffffc
6: 48 8b 05 00 00 00 00 mov 0(%rip),%rax # d <foo1+0xd>
9: R_X86_64_PC32 table+0xfffffffffffffffc
d: d3 ef shr %cl,%edi
f: 89 ff mov %edi,%edi
11: 8b 04 b8 mov (%rax,%rdi,4),%eax
14: c1 e8 08 shr $0x8,%eax
17: c3 retq
the "mov %edi,%edi" is presumably there as a 32->64 extend... but it's not
necessary because %edi was written in previous instruction.
% gcc -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc/configure --prefix=/home/odo/gcc --disable-multilib
--disable-biarch x86_64-unknown-linux-gnu --enable-languages=c
Thread model: posix
gcc version 4.3.0 20070217 (experimental)
i've seen it in 4.1.1 too.
-dean
--
Summary: extra register move to self
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: dean at arctic dot org
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=30829
