From cc9e401e3cf11bf353bf7b2734f89fcb74f16d95 Mon Sep 17 00:00:00 2001 From: Sanjay Patel Date: Fri, 26 Oct 2018 21:05:14 +0000 Subject: [ValueTracking] peek through shuffles in ComputeNumSignBits (PR37549) The motivating case is from PR37549: https://bugs.llvm.org/show_bug.cgi?id=37549 The analysis improvement allows us to form a vector 'select' out of bitwise logic (the use of ComputeNumSignBits was added at rL345149). The smaller test shows another InstCombine improvement - we use ComputeNumSignBits to add 'nsw' to shift-left. But the negative test shows an example where we must not add 'nsw' - when the shuffle mask contains undef elements. Differential Revision: https://reviews.llvm.org/D53659 llvm-svn: 345429 --- llvm/lib/Analysis/ValueTracking.cpp | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) (limited to 'llvm/lib/Analysis/ValueTracking.cpp') diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp index b7ff81f..3cef373 100644 --- a/llvm/lib/Analysis/ValueTracking.cpp +++ b/llvm/lib/Analysis/ValueTracking.cpp @@ -2510,6 +2510,27 @@ static unsigned ComputeNumSignBitsImpl(const Value *V, unsigned Depth, // valid for all elements of the vector (for example if vector is sign // extended, shifted, etc). return ComputeNumSignBits(U->getOperand(0), Depth + 1, Q); + + case Instruction::ShuffleVector: + // If the shuffle mask contains any undefined elements, that element of the + // result is undefined. Propagating information from a source operand may + // not be correct in that case, so just bail out. + if (cast(U)->getMask()->containsUndefElement()) + break; + + assert((!isa(U->getOperand(0)) || + !isa(U->getOperand(1))) + && "Should have simplified shuffle with 2 undef inputs"); + + // Look through shuffle of 1 source vector. + if (isa(U->getOperand(0))) + return ComputeNumSignBits(U->getOperand(1), Depth + 1, Q); + if (isa(U->getOperand(1))) + return ComputeNumSignBits(U->getOperand(0), Depth + 1, Q); + + // TODO: We can look through shuffles of 2 sources by computing the minimum + // sign bits for each operand (similar to what we do for binops). + break; } // Finally, if we can prove that the top bits of the result are 0's or 1's, -- cgit v1.1