[FuncSpec] Return changed if function is changed by tryToReplaceWithConstant

The may get changed before specialization by RunSCCPSolver. In other
words, the pass may change the function without specialization happens.
Add test and comment to reveal this.
And it may return No Changed if the function get changed by
RunSCCPSolver before the specialization. It looks like a potential bug.

Test Plan: check-all

Reviewed By: https://reviews.llvm.org/D107622

Differential Revision: https://reviews.llvm.org/D107622

llvm/lib/Transforms/IPO/FunctionSpecialization.cpp

@@ -26,6 +26,8 @@
 // `func-specialization-max-iters`
 //   increases linearly. See discussion in https://reviews.llvm.org/D106426 for
 //   details.
+// - Don't transform the function if there is no function specialization
+// happens.
 //
 //===----------------------------------------------------------------------===//
 
@@ -760,7 +762,9 @@ bool llvm::runFunctionSpecialization(
         if (!Solver.isBlockExecutable(&BB))
           continue;
         for (auto &I : make_early_inc_range(BB))
-          FS.tryToReplaceWithConstant(&I);
+          // FIXME: The solver may make changes to the function here, so set Changed, even if later
+          // function specialization does not trigger.
+          Changed |= FS.tryToReplaceWithConstant(&I);
       }
     }
   };

llvm/test/Transforms/FunctionSpecialization/function-specialization-constant-expression.ll

@@ -0,0 +1,49 @@
+; Test function specialization wouldn't crash due to constant expression.
+; Note that this test case shows that function specialization pass would
+; transform the function even if no specialization happened.
+; RUN: opt -function-specialization -S < %s | FileCheck %s
+
+; CHECK: plus:
+; CHECK-NEXT:  %{{.*}} = call i64 @func2(i64* getelementptr inbounds (%struct, %struct* @Global, i32 0, i32 3))
+; CHECK: minus:
+; CHECK-NEXT: %{{.*}} = call i64 @func2(i64* getelementptr inbounds (%struct, %struct* @Global, i32 0, i32 4))
+
+%struct = type { i8, i16, i32, i64, i64}
+@Global = internal constant %struct {i8 0, i16 1, i32 2, i64 3, i64 4}
+define internal i64 @zoo(i1 %flag) {
+entry:
+  br i1 %flag, label %plus, label %minus
+
+plus:
+  %arg = getelementptr %struct, %struct* @Global, i32 0, i32 3
+  %tmp0 = call i64 @func2(i64* %arg)
+  br label %merge
+
+minus:
+  %arg2 = getelementptr %struct, %struct* @Global, i32 0, i32 4
+  %tmp1 = call i64 @func2(i64* %arg2)
+  br label %merge
+
+merge:
+  %tmp2 = phi i64 [ %tmp0, %plus ], [ %tmp1, %minus]
+  ret i64 %tmp2
+}
+
+define internal i64 @func2(i64 *%x) {
+entry:
+  %val = ptrtoint i64* %x to i64
+  ret i64 %val
+}
+
+define internal i64 @func(i64 *%x, i64 (i64*)* %binop) {
+entry:
+  %tmp0 = call i64 %binop(i64* %x)
+  ret i64 %tmp0
+}
+
+define i64 @main() {
+    %1 = call i64 @zoo(i1 0)
+    %2 = call i64 @zoo(i1 1)
+    %3 = add i64 %1, %2
+    ret i64 %3
+}