[InstCombine] Optimize icmp slt signum(x), 1 --> icmp slt x, 1

Summary:
`signum(x)` is sometimes implemented as `(x >> 63) | (-x >>> 63)` (for
an `i64` `x`).  This change adds a matcher for that pattern, and an
instcombine rule to optimize `signum(x) s< 1`.

Later, we can also consider optimizing:

  icmp slt signum(x), 0 --> icmp slt x, 0
  icmp sle signum(x), 1 --> true

etc.

Reviewers: majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12703

llvm-svn: 247846

1 files changed, 17 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index f1a66816c4c..0d56ce5ebe8 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1145,6 +1145,14 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
 
   switch (LHSI->getOpcode()) {
   case Instruction::Trunc:
+    if (RHS->isOne() && RHSV.getBitWidth() > 1) {
+      // icmp slt trunc(signum(V)) 1 --> icmp slt V, 1
+      Value *V = nullptr;
+      if (ICI.getPredicate() == ICmpInst::ICMP_SLT &&
+          match(LHSI->getOperand(0), m_Signum(m_Value(V))))
+        return new ICmpInst(ICmpInst::ICMP_SLT, V,
+                            ConstantInt::get(V->getType(), 1));
+    }
     if (ICI.isEquality() && LHSI->hasOneUse()) {
       // Simplify icmp eq (trunc x to i8), 42 -> icmp eq x, 42|highbits if all
       // of the high bits truncated out of x are known.
@@ -1467,6 +1475,15 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
     break;
 
   case Instruction::Or: {
+    if (RHS->isOne()) {
+      // icmp slt signum(V) 1 --> icmp slt V, 1
+      Value *V = nullptr;
+      if (ICI.getPredicate() == ICmpInst::ICMP_SLT &&
+          match(LHSI, m_Signum(m_Value(V))))
+        return new ICmpInst(ICmpInst::ICMP_SLT, V,
+                            ConstantInt::get(V->getType(), 1));
+    }
+
     if (!ICI.isEquality() || !RHS->isNullValue() || !LHSI->hasOneUse())
       break;
     Value *P, *Q;