[LLVMbugs] [Bug 18602] New: A move to a specific target register is often not optimized away

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18602

            Bug ID: 18602
           Summary: A move to a specific target register is often not
                    optimized away
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: jn at sirrida.de
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

A move to a specific target register is often not optimized away.
This unnecessarily costs 2 bytes code:

int test(int x) {
  return ((x>>2) & 15) ^ ((x>>3) & 31);
  }
=> clang -O3
    movl    %edi, %eax
    shrl    $2, %eax
    andl    $15, %eax
    shrl    $3, %edi
    andl    $31, %edi
    xorl    %eax, %edi  //
    movl    %edi, %eax  // could become xorl %edi, %eax
    retq  // we need %eax here

int test(int x, int y) {
  return (((x>>2) & 15) ^ ((x<<3) & ~31)) / y;
  }
=> clang -O3
    movl    %edi, %eax
    shrl    $2, %eax
    andl    $15, %eax
    shll    $3, %edi
    andl    $-32, %edi
    orl    %eax, %edi  //
    movl    %edi, %eax  // could become orl %edi, %eax
    cltd  // we need %eax here
    idivl    %esi
    retq

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