[InstCombine] foldICmpWithLowBitMaskedVal(): handle ~(-1 << y) mask

Summary:
Two folds are happening here:
1. https://rise4fun.com/Alive/oaFX
2. And then `foldICmpWithHighBitMask()` (D52001): https://rise4fun.com/Alive/wsP4

This change doesn't just add the handling for eq/ne predicates,
it actually builds upon the previous `foldICmpWithLowBitMaskedVal()` work,
so **all** the 16 fold variants* are immediately supported.

I'm indeed only testing these two predicates.
I do not feel like re-proving all 16 folds*, because they were already proven
for the general case of constant with all-ones in low bits. So as long as
the mask produces all-ones in low bits, i'm pretty sure the fold is valid.

But required, i can re-prove, let me know.

* eq/ne are commutative - 4 folds; ult/ule/ugt/uge - are not commutative (the commuted variant is InstSimplified), 4 folds; slt/sle/sgt/sge are not commutative - 4 folds. 12 folds in total.

https://bugs.llvm.org/show_bug.cgi?id=38123
https://bugs.llvm.org/show_bug.cgi?id=38708

Reviewers: spatel, craig.topper, RKSimon

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52146

llvm-svn: 342546

1 files changed, 7 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 5f8bf84092c..f06a0e83c9b 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2883,7 +2883,10 @@ Instruction *InstCombiner::foldICmpInstWithConstantNotInt(ICmpInst &I) {
 /// In this case, we are looking for comparisons that look like
 /// a check for a lossy truncation.
 /// Folds:
-///   x & (-1 >> y) SrcPred x    to    x DstPred (-1 >> y)
+///   icmp SrcPred (x & Mask), x    to    icmp DstPred x, Mask
+/// Where Mask is some pattern that produces all-ones in low bits:
+///    (-1 >> y)
+///   ~(-1 << y)
 /// The Mask can be a constant, too.
 /// For some predicates, the operands are commutative.
 /// For others, x can only be on a specific side.
@@ -2891,7 +2894,9 @@ static Value *foldICmpWithLowBitMaskedVal(ICmpInst &I,
                                           InstCombiner::BuilderTy &Builder) {
   ICmpInst::Predicate SrcPred;
   Value *X, *M;
-  auto m_Mask = m_CombineOr(m_LShr(m_AllOnes(), m_Value()), m_LowBitMask());
+  auto m_VariableMask = m_CombineOr(m_Not(m_Shl(m_AllOnes(), m_Value())),
+                                    m_LShr(m_AllOnes(), m_Value()));
+  auto m_Mask = m_CombineOr(m_VariableMask, m_LowBitMask());
   if (!match(&I, m_c_ICmp(SrcPred,
                           m_c_And(m_CombineAnd(m_Mask, m_Value(M)), m_Value(X)),
                           m_Deferred(X))))