Index: llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp
+++ llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp
@@ -220,6 +220,9 @@
   // TODO: Consider accumulating the offset to the base.
   AAPtr = AAPtr->stripPointerCastsSameRepresentation();
   AlignSCEV = SE->getSCEV(AlignOB.Inputs[1].get());
+  // We only handle a simple constant (constant expressions not allowed).
+  if (!isa<SCEVConstant>(AlignSCEV))
+    return false;
   AlignSCEV = SE->getTruncateOrZeroExtend(AlignSCEV, Int64Ty);
   if (AlignOB.Inputs.size() == 3)
     OffSCEV = SE->getSCEV(AlignOB.Inputs[2].get());
Index: llvm/test/Transforms/AlignmentFromAssumptions/simple.ll
===================================================================
--- llvm/test/Transforms/AlignmentFromAssumptions/simple.ll
+++ llvm/test/Transforms/AlignmentFromAssumptions/simple.ll
@@ -252,6 +252,21 @@
 ; CHECK: ret i32 undef
 }
 
+; Don't crash on constant expression.
+
+@g = global i32 0
+
+define void @PR50978(i32* %arg) {
+; CHECK-LABEL: @PR50978(
+; CHECK-NEXT:    call void @llvm.assume(i1 true) [ "align"(i32* [[ARG:%.*]], i64 zext (i32 ptrtoint (i32* @g to i32) to i64)) ]
+; CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* [[ARG]], align 4
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.assume(i1 true) [ "align"(i32* %arg, i64 zext (i32 ptrtoint (i32* @g to i32) to i64)) ]
+  load i32, i32* %arg
+  ret void
+}
+
 declare void @llvm.assume(i1) nounwind
 
 declare void @llvm.memset.p0i8.i64(i8* nocapture, i8, i64, i1) nounwind