Explicitly handle pointer comparisons

These don't get handled by the m_APInt code.

Author: nikic

llvm/lib/Analysis/ValueTracking.cpp

@@ -629,6 +629,28 @@ static bool isKnownNonZeroFromAssume(const Value *V, const SimplifyQuery &Q) {
 static void computeKnownBitsFromCmp(const Value *V, const ICmpInst *Cmp,
                                     KnownBits &Known, unsigned Depth,
                                     const SimplifyQuery &Q) {
+  if (Cmp->getOperand(1)->getType()->isPointerTy()) {
+    // Handle comparison of pointer to null explicitly, as it will not be
+    // covered by the m_APInt() logic below.
+    if (match(Cmp->getOperand(1), m_Zero())) {
+      switch (Cmp->getPredicate()) {
+      case ICmpInst::ICMP_EQ:
+        Known.setAllZero();
+        break;
+      case ICmpInst::ICMP_SGE:
+      case ICmpInst::ICMP_SGT:
+        Known.makeNonNegative();
+        break;
+      case ICmpInst::ICMP_SLT:
+        Known.makeNegative();
+        break;
+      default:
+        break;
+      }
+    }
+    return;
+  }
+
   unsigned BitWidth = Known.getBitWidth();
   auto m_V =
       m_CombineOr(m_Specific(V), m_PtrToIntSameSize(Q.DL, m_Specific(V)));

llvm/test/Transforms/InstCombine/icmp.ll

@@ -4016,10 +4016,9 @@ define i32 @abs_preserve(i32 %x) {
 declare void @llvm.assume(i1)
 define i1 @PR35794(ptr %a) {
 ; CHECK-LABEL: @PR35794(
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt ptr [[A:%.*]], inttoptr (i64 -1 to ptr)
-; CHECK-NEXT:    [[MASKCOND:%.*]] = icmp eq ptr [[A]], null
+; CHECK-NEXT:    [[MASKCOND:%.*]] = icmp eq ptr [[A:%.*]], null
 ; CHECK-NEXT:    tail call void @llvm.assume(i1 [[MASKCOND]])
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 true
 ;
   %cmp = icmp sgt ptr %a, inttoptr (i64 -1 to ptr)
   %maskcond = icmp eq ptr %a, null