http://llvm.org/bugs/show_bug.cgi?id=21993
Bug ID: 21993
Summary: Compiler missing corner case of single bit test
optimization
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: Common Code Generator Code
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
In r222871, an optimization was re-added to optimize sequences of the form:
(X & Y) == 0 ? X : X ^ Y --> X & ~Y
(X & Y) != 0 ? X ^ Y : X --> X & ~Y
(X & Y) == 0 ? X ^ Y : X --> X | Y
(X & Y) != 0 ? X : X ^ Y --> X | Y
It seems to work for most cases, but it seems if Y=0x80000000, the optimization
is not applied when it could be.
Consider the following code:
=====
unsigned int test0a(unsigned int x) {
return (x & CONSTANT) == 0 ? x : (x ^ CONSTANT);
}
-- or --
unsigned int test0b(unsigned int x) {
if((x & CONSTANT) == 0)
return x;
else
return (x ^ CONSTANT);
}
=====
Both represent the first pattern. If they are compiled targeting x64 linux with
-O1 and Y=0x8, clang (r224566) will generate the following code for both
functions:
andl $-9, %edi
movl %edi, %eax
which is what we expect.
However, if we use the same compiler and instead use Y=0x80000000, the compiler
generates the following for the functions:
movl %edi, %eax
xorl $-2147483648, %eax # imm = 0xFFFFFFFF80000000
testl %edi, %edi
cmovnsl %edi, %eax
The code is still correct, however, it could be better. The optimization
re-added with r222871 does not seem to be detecting this case as valid for
optimization when it should.
--
You are receiving this mail because:
You are on the CC list for the bug.
_______________________________________________
LLVMbugs mailing list
[email protected]
http://lists.cs.uiuc.edu/mailman/listinfo/llvmbugs
