[LLVMbugs] [Bug 16938] New: Duplicate loading of double constants

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=16938

            Bug ID: 16938
           Summary: Duplicate loading of double constants
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: eltoder at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

I found that in some cases llvm generates duplicate loads of double constants,
e.g.

$ cat t.c
double f(double* p, int n)
{
    double s = 0;
    if (n)
        s += *p;
    return s;
}
$ clang -S -O3 t.c -o -
...
f:                                      # @f
        .cfi_startproc
# BB#0:
        xorps   %xmm0, %xmm0
        testl   %esi, %esi
        je      .LBB0_2
# BB#1:
        xorps   %xmm0, %xmm0
        addsd   (%rdi), %xmm0
.LBB0_2:
        ret
...

Note that there are 2 xorps instructions, the one in BB#1 being clearly
redundant
as it's dominated by the first one. Two xorps come from 2 FsFLD0SD generated by
instruction selection and never eliminated by machine passes. My guess would be
machine CSE should take care of it.

A variation of this case without indirection has the same problem, as well as a
related but separate issue: not commuting addps, resulting in an extra movps:

$ cat t.c
double f(double p, int n)
{
    double s = 0;
    if (n)
        s += p;
    return s;
}
$ clang -S -O3 t.c -o -
...
f:                                      # @f
        .cfi_startproc
# BB#0:
        xorps   %xmm1, %xmm1
        testl   %edi, %edi
        je      .LBB0_2
# BB#1:
        xorps   %xmm1, %xmm1
        addsd   %xmm1, %xmm0
        movaps  %xmm0, %xmm1
.LBB0_2:
        movaps  %xmm1, %xmm0
        ret
...

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