in the following code, the compiler zero-extends the same byte twice... there's
something about using byte0 in an if () statement which triggers this -- if you
comment out the if() the problem goes away.
-dean
% cat redundant_movzbl.c
unsigned foo(unsigned char *p)
{
unsigned byte0 = *p;
unsigned len = __builtin_ctz(byte0) + 1;
if (byte0 == 0) return 0;
return len;
}
% ~/gcc/bin/gcc -g -O3 -Wall -c redundant_movzbl.c
% objdump -dr redundant_movzbl.o
redundant_movzbl.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <foo>:
0: 0f b6 07 movzbl (%rdi),%eax
3: 31 d2 xor %edx,%edx
5: 84 c0 test %al,%al
7: 74 09 je 12 <foo+0x12>
9: 0f b6 c0 movzbl %al,%eax
c: 0f bc c0 bsf %eax,%eax
f: 8d 50 01 lea 0x1(%rax),%edx
12: 89 d0 mov %edx,%eax
14: c3 retq
% ~/gcc/bin/gcc -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc/configure --prefix=/home/odo/gcc --enable-languages=c
--enable-targets=x86_64-unknown-linux-gnu x86_64-unknown-linux-gnu :
(reconfigured) ../gcc/configure --prefix=/home/odo/gcc --enable-languages=c
--without-mudflap --disable-biarch x86_64-unknown-linux-gnu : (reconfigured)
../gcc/configure --prefix=/home/odo/gcc --enable-languages=c --disable-multilib
--disable-biarch x86_64-unknown-linux-gnu
Thread model: posix
gcc version 4.3.0 20061104 (experimental)
--
Summary: redundant movzbl
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=29775
