http://llvm.org/bugs/show_bug.cgi?id=22788
Bug ID: 22788
Summary: Redundant zeroing of high vector elements for scalar
SSE/AVX instructions
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Using scalar x86 intrinsics results in IR similar to this:
define void @foo(double* %p, double* %q, <2 x double>* %r) {
%v = load double, double* %p, align 16
%w = load double, double* %q, align 16
%mul = fmul double %v, %w
%vec = insertelement <2 x double> <double undef, double 0.000000e+00>, double
%mul, i32 0
store <2 x double> %vec, <2 x double>* %r, align 16
ret void
}
This gets lowered into:
movsd (%rcx), %xmm0 # xmm0 = mem[0],zero
mulsd (%rdx), %xmm0
movq %xmm0, %xmm0 # xmm0 = xmm0[0],zero
movdqa %xmm0, (%r8)
The movq is redundant - movsd from memory zeroes the high element, and mulsd
preserves it.
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