[LLVMbugs] [Bug 18604] New: The register allocator allocates unnecessarily many registers

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18604

            Bug ID: 18604
           Summary: The register allocator allocates unnecessarily many
                    registers
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: jn at sirrida.de
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The register allocator allocates unnecessarily many registers.
Trying to minimize the number of involved registers allows for
more freely usable registers in the callers, assuming inlining is too costly.

int test(int x) {
  int y;
  x ^= (x >> 2);
  x = (x >> 3) ^ x;
  x = x ^ (x >> 4);
  y = x;  x >>= 5;  x ^= y;  // almost the same but explicit
  return x;
  }
=>
    movl    %edi, %eax
    sarl    $2, %eax
    xorl    %edi, %eax
    movl    %eax, %ecx  // %ecx => %edi
    sarl    $3, %ecx
    xorl    %eax, %ecx
    movl    %ecx, %edx  // %ecx => %edi; %edx => %eax
    sarl    $4, %edx
    xorl    %ecx, %edx
    movl    %edx, %eax  // %eax => %edi; %edx => %eax
    sarl    $5, %eax
    xorl    %edx, %eax  // reverse operands
    retq

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