https://bugs.llvm.org/show_bug.cgi?id=36760
Bug ID: 36760
Summary: Arithmetic shift optimization for signed numbers
Product: libraries
Version: 6.0
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: nruslan_de...@yahoo.com
CC: llvm-bugs@lists.llvm.org
This was reported a while ago, but seems clang/llvm still does not optimize
this case well. (For the examples below -O2 option is used)
1. Consider the following example:
long long func(long long a)
{
if (a < 0)
return 0;
return (a >> 63 ) ^ a;
}
Basically, since a is already checked to be non-negative, (a >> 63) must be 0.
Therefore, the shift operation should be optimized out.
clang/llvm 6.0 generates the following:
movq %rdi, %rcx
sarq $63, %rcx
xorq %rdi, %rcx
xorl %eax, %eax
testq %rdi, %rdi
cmovnsq %rcx, %rax
retq
whereas gcc can optimize it well:
testq %rdi, %rdi
movl $0, %eax
cmovns %rdi, %rax
ret
2. Another example is when we reverse the condition:
long long func(long long a)
{
if (a >= 0)
return 0;
return (a >> 63 ) ^ a;
}
In this case, again, the shift is unnecessary since (a >> 63) are all 1's.
Therefore, return statement is basically ~a.
clang/llvm:
movq %rdi, %rcx
sarq $63, %rcx
xorq %rdi, %rcx
xorl %eax, %eax
testq %rdi, %rdi
cmovsq %rcx, %rax
retq
gcc:
movq %rdi, %rax
testq %rdi, %rdi
movl $0, %edx
notq %rax
cmovns %rdx, %rax
ret
--
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