https://llvm.org/bugs/show_bug.cgi?id=31880
Bug ID: 31880
Summary: shuffle and vectorize repeated scalar ops on extracted
elements
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Transformation Utilities
Assignee: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Forking this off from the (possibly more specific and less useful) bug 31879
and the motivating (complex numbers) bug 31866:
; For vectors <x0, x1> and <y0, y1>, return <x0*x0, y1*y1>
define <2 x i8> @g(<2 x i8> %x, <2 x i8> %y) {
%x0 = extractelement <2 x i8> %x, i32 0
%y1 = extractelement <2 x i8> %y, i32 1
%x0x0 = mul i8 %x0, %x0
%y1y1 = mul i8 %y1, %y1
%ins1 = insertelement <2 x i8> undef, i8 %x0x0, i32 0
%ins2 = insertelement <2 x i8> %ins1, i8 %y1y1, i32 1
ret <2 x i8> %ins2
}
The canonical IR should be:
define <2 x i8> @h(<2 x i8> %x, <2 x i8> %y) {
%x0y1 = shufflevector <2 x i8> %x, <2 x i8> %y, <2 x i32> <i32 0, i32 3>
%x0x0y1y1 = mul <2 x i8> %x0y1, %x0y1
ret <2 x i8> %x0x0y1y1
}
This is obviously less instructions and does no extra computational work. Ie,
there are still just two 8-bit multiplies, and we're not operating on unknown
vector elements. Therefore, this is safe even for FP vectors that might contain
perf bombs like denorms because we're still not going to touch them.
The backend will scalarize the vector op if it is not supported to effectively
undo this transform. That should also eliminate the shuffle.
Note that we don't create arbitrary shuffles in IR because it could be
disastrous for the backend, but this is a special case: it's a "blend" (x86
terminology). Ie, we're taking elements from the 2 source vectors without
crossing vector lanes. There's precedent for this type of shuffle because we
canonicalize vector selects to this form:
define <2 x i8> @h(<2 x i8> %x, <2 x i8> %y) {
%x0y1 = select <2 x i1> <i1 true, i1 false>, <2 x i8> %x, <2 x i8> %y
%mul = mul <2 x i8> %x0y1, %x0y1
ret <2 x i8> %mul
}
$ ./opt -instcombine scalarizedmath.ll -S
define <2 x i8> @h(<2 x i8> %x, <2 x i8> %y) {
%x0y1 = shufflevector <2 x i8> %x, <2 x i8> %y, <2 x i32> <i32 0, i32 3>
%mul = mul <2 x i8> %x0y1, %x0y1
ret <2 x i8> %mul
}
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