Automerge: [clang] Convert second arg of __builtin_assume_aligned to ConstantExpr (#161314)

Since the second argument must be a constant integer, we can as well
convert it to a `ConstantExpr` in Sema.

Fixes llvm/llvm-project#161272

Author: tbaederr

clang/docs/ReleaseNotes.rst

@@ -246,6 +246,8 @@ Non-comprehensive list of changes in this release
 
 - ``__builtin_assume_dereferenceable`` now accepts non-constant size operands.
 
+- Fixed a crash when the second argument to ``__builtin_assume_aligned`` was not constant (#GH161314)
+
 New Compiler Flags
 ------------------
 - New option ``-fno-sanitize-debug-trap-reasons`` added to disable emitting trap reasons into the debug info when compiling with trapping UBSan (e.g. ``-fsanitize-trap=undefined``).

clang/lib/Sema/SemaChecking.cpp

@@ -5954,6 +5954,9 @@ bool Sema::BuiltinAssumeAligned(CallExpr *TheCall) {
     if (Result > Sema::MaximumAlignment)
       Diag(TheCall->getBeginLoc(), diag::warn_assume_aligned_too_great)
           << SecondArg->getSourceRange() << Sema::MaximumAlignment;
+
+    TheCall->setArg(1,
+                    ConstantExpr::Create(Context, SecondArg, APValue(Result)));
   }
 
   if (NumArgs > 2) {

clang/test/SemaCXX/builtin-assume-aligned.cpp

@@ -47,3 +47,9 @@ constexpr void *s1 = __builtin_assume_aligned(x, 32);
 constexpr void *s2 = __builtin_assume_aligned(x, 32, 5);
 constexpr void *s3 = __builtin_assume_aligned(x, 32, -1);
 
+
+constexpr int add(int a, int b) {
+  return a+b;
+}
+constexpr void *c1 = __builtin_assume_aligned(p, add(1,1));
+constexpr void *c2 = __builtin_assume_aligned(p, add(2,1)); // expected-error {{not a power of 2}}