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: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: llvm-bugs@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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