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  1. [PowerPC] Fix MI peephole optimization for splats (details)
Commit 884351547da27e76e14bef5fe20c3e3cb0e89acd by lkail
[PowerPC] Fix MI peephole optimization for splats
Summary: This patch fixes an issue where the PPC MI peephole
optimization pass incorrectly remove a vector swap.
Specifically, the pass can combine a splat/swap to a splat/copy. It uses
`TargetRegisterInfo::lookThruCopyLike` to determine that the operands to
the splat are the same. However, the current logic only compares the
operands based on register numbers. In the case where the splat operands
are ultimately feed from the same physical register, the pass can
incorrectly remove a swap if the feed register for one of the operands
has been clobbered.
This patch adds a check to ensure that the registers feeding are both
virtual registers or the operands to the splat or swap are both the same
Here is an example in pseudo-MIR of what happens in the test cased added
in this patch:
Before PPC MI peephole optimization:
%arg = XVADDDP %0, %1
$f1 = COPY %arg.sub_64 call double rint(double)
%res.first = COPY $f1
%vec.res.first = SUBREG_TO_REG 1, %res.first, %subreg.sub_64
%arg.swapped = XXPERMDI %arg, %arg, 2
$f1 = COPY %arg.swapped.sub_64 call double rint(double)
%res.second = COPY $f1
%vec.res.second = SUBREG_TO_REG 1, %res.second, %subreg.sub_64
%vec.res.splat = XXPERMDI %vec.res.first, %vec.res.second, 0
%vec.res = XXPERMDI %vec.res.splat, %vec.res.splat, 2
; %vec.res == [ %vec.res.second[0], %vec.res.first[0] ]
After optimization:
; ...
%vec.res.splat = XXPERMDI %vec.res.first, %vec.res.second, 0
; lookThruCopyLike(%vec.res.first) == lookThruCopyLike(%vec.res.second)
== $f1
; so the pass replaces the swap with a copy:
%vec.res = COPY %vec.res.splat
; %vec.res == [ %vec.res.first[0], %vec.res.second[0] ]
As best as I can tell, this has occurred since r288152, which added
support for lowering certain vector operations to direct moves in the
form of a splat.
Committed for vddvss (Colin Samples). Thanks Colin for the patch!
Differential Revision:
The file was modifiedllvm/lib/Target/PowerPC/PPCMIPeephole.cpp
The file was addedllvm/test/CodeGen/PowerPC/mi-peephole-splat.ll
The file was modifiedllvm/test/CodeGen/PowerPC/vector-constrained-fp-intrinsics.ll