array[(x>>2)&3] can be arranged to avoid the right shift.
consider:
% cat shift-and-fold.c
unsigned array[4];
unsigned foo(unsigned long x)
{
return array[(x>>2)&3ul];
}
% /home/odo/gcc/bin/gcc -g -O3 -Wall -c -o shift-and-fold.o shift-and-fold.c
% objdump -dr shift-and-fold.o
shift-and-fold.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <foo>:
0: 48 c1 ef 02 shr $0x2,%rdi
4: 83 e7 03 and $0x3,%edi
7: 8b 04 bd 00 00 00 00 mov 0x0(,%rdi,4),%eax
a: R_X86_64_32S array
e: c3 retq
could be:
and $0xc,%edi
mov 0(%rdi),%eax
gcc does the right thing when the left shift isn't part of an address...
thanks
-dean
% /home/odo/gcc/bin/gcc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../gcc/configure --prefix=/home/odo/gcc
--host=x86_64-linux-gnu --disable-multilib --enable-languages=c,c++
--disable-bootstrap
Thread model: posix
gcc version 4.2.0 20060603 (experimental)
--
Summary: eliminate shift in array[(x>>2)&3]
Product: gcc
Version: 4.2.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: pending
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: dean at arctic dot org
GCC build triplet: x86_64-linux-gnu
GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27971
