[llvm-bugs] [Bug 26859] New: [x86, SSE] phaddw / phaddd wrongly generated

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26859

            Bug ID: 26859
           Summary: [x86, SSE] phaddw / phaddd wrongly generated
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Packed horizontal add - phaddd / phaddw:
These are SSSE3 (yes, 3 Ss) instructions that should probably never be
generated for performance reasons, only to save on code size. They're just
about guaranteed to be slow because they operate across vector lanes.

Here we're not only generating these things, but for a target that doesn't have
SSSE3:

$ cat accum.c 
int please_no_phaddd(int *x) {
  int sum = 0;
  for (int i=0; i<1024; ++i)
    sum += x[i];
  return sum;
}

short please_no_phaddw(short *x) {
  short sum = 0;
  for (int i=0; i<1024; ++i)
    sum += x[i];
  return sum;
}
bin $ ./clang -O2 -S -o - accum.c -msse -fno-unroll-loops|grep phadd
    .globl    _please_no_phaddd
_please_no_phaddd:                      ## @please_no_phaddd
    phaddd    %xmm1, %xmm1
    .globl    _please_no_phaddw
_please_no_phaddw:                      ## @please_no_phaddw
    phaddw    %xmm0, %xmm0

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