http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60641
Bug ID: 60641
Summary: Converting ushort to offset on x86_64 generates double
movzwl
Product: gcc
Version: 4.7.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: akruppa at gmail dot com
The following test cases produce sub-optimal assembly output.
This was verified with these three versions of gcc:
gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
gcc (SUSE Linux) 4.7.3
gcc (Debian 4.8.2-16) 4.8.2
#include <stdlib.h>
unsigned short
foo (const unsigned short *start, const unsigned char *mask)
{
unsigned short r = 0;
unsigned short ux = *start;
if (mask[ux])
r = ux;
return r;
}
void bar(unsigned int *s, unsigned short a)
{
s[a] = a;
}
Compile with, e.g.,
gcc -std=c99 -g -W -Wall -O3 -c movzwl.c
The effect also occurs when using -O2 instead.
The foo() function contains:
0x0000000000000000 <+0>: movzwl (%rdi),%edx
0x0000000000000003 <+3>: xor %eax,%eax
0x0000000000000005 <+5>: movzwl %dx,%ecx
0x0000000000000008 <+8>: cmpb $0x0,(%rsi,%rcx,1)
The foo() function is:
0x0000000000000010 <+0>: movzwl %si,%eax
0x0000000000000013 <+3>: movzwl %si,%esi
0x0000000000000016 <+6>: mov %esi,(%rdi,%rax,4)
0x0000000000000019 <+9>: retq
In both cases, an unnecessary movzwl instruction is generated. This may be the
same issue as #36873, which was rejected because the test cases used volatile
accesses. These test cases here show that the duplicate movzwl occurs without
volatile as well.
