[LLVMbugs] [Bug 11703] New: poor x86-64 register allocation

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Wed Jan 4 04:19:59 PST 2012


             Bug #: 11703
           Summary: poor x86-64 register allocation
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Register Allocator
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: jay.foad at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

I get:

$ echo "unsigned f(unsigned x) { return (x >> 1) + (x & 1); }" | clang -O3 -S
-o - -x c -
# BB#0:                                 # %entry
    movl    %edi, %eax
    andl    $1, %eax
    shrl    %edi
    addl    %eax, %edi
    movl    %edi, %eax

This seems like poor register allocation, because the last two instructions
before the ret could be replaced with:

    addl    %edi, %eax

I'm using trunk rev 147529 on Ubuntu 11.10 x86-64.

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