[BasicAA] Check nusw instead of inbounds

For the offset scaling, this is sufficient to guarantee nsw. The
other checks for inbounds in this file do need proper inbounds.

Author: nikic

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -701,7 +701,7 @@ BasicAAResult::DecomposeGEPExpression(const Value *V, const DataLayout &DL,
 
       // Scale by the type size.
       unsigned TypeSize = AllocTypeSize.getFixedValue();
-      LE = LE.mul(APInt(IndexSize, TypeSize), GEPOp->isInBounds());
+      LE = LE.mul(APInt(IndexSize, TypeSize), GEPOp->hasNoUnsignedSignedWrap());
       Decomposed.Offset += LE.Offset.sext(MaxIndexSize);
       APInt Scale = LE.Scale.sext(MaxIndexSize);
 

llvm/test/Analysis/BasicAA/struct-geps.ll

@@ -51,6 +51,17 @@ define void @test_not_inbounds(ptr %st, i64 %i, i64 %j, i64 %k) {
   ret void
 }
 
+; It is sufficient to have nusw instead of inbounds.
+; CHECK-LABEL: test_nusw
+; CHECK: NoAlias: i32* %x, i32* %y
+define void @test_nusw(ptr %st, i64 %i, i64 %j, i64 %k) {
+  %x = getelementptr nusw %struct, ptr %st, i64 %i, i32 0
+  %y = getelementptr nusw %struct, ptr %st, i64 %j, i32 1
+  load i32, ptr %x
+  load i32, ptr %y
+  ret void
+}
+
 ; CHECK-LABEL: test_in_array
 
 ; CHECK-DAG: MayAlias: [1 x %struct]* %st, i32* %x