[clang][ExprConstant] Fix assertion failure in constant expression folding

[antmo]

Summary

Since D140059, clang asserts on constant calls of string functions
(memchr, strncmp, memcmp, ...) when the last argument (length) does not
fit in an UInt63:

strncmp("11", "12", -1);

Fix this by directly calling getZExtValue, as an unsigned value is
needed here

[antmo]

rebase & ping ; )

[shafik]

It looks like I fixed this in: https://reviews.llvm.org/D158557

[antmo]

yes indeed you fixed this @shafik. thanks for the update. abandoning this revision

clang/lib/AST/ExprConstant.cpp

 //===--- ExprConstant.cpp - Expression Constant Evaluator -----------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
case Builtin::BI__builtin_wmemchr: {
     uint64_t MaxLength = uint64_t(-1);
     if (BuiltinOp != Builtin::BIstrchr &&
         BuiltinOp != Builtin::BIwcschr &&
         BuiltinOp != Builtin::BI__builtin_strchr &&
         BuiltinOp != Builtin::BI__builtin_wcschr) {
       APSInt N;
       if (!EvaluateInteger(E->getArg(2), N, Info))
         return false;
-      MaxLength = N.getExtValue();
+      MaxLength = N.getZExtValue();
     }
     // We cannot find the value if there are no candidates to match against.
     if (MaxLength == 0u)
       return ZeroInitialization(E);
     if (!Result.checkNullPointerForFoldAccess(Info, E, AK_Read) ||
         Result.Designator.Invalid)
       return false;
     QualType CharTy = Result.Designator.getType(Info.Ctx);
case Builtin::BI__builtin_wmemcmp: {
     uint64_t MaxLength = uint64_t(-1);
     if (BuiltinOp != Builtin::BIstrcmp &&
         BuiltinOp != Builtin::BIwcscmp &&
         BuiltinOp != Builtin::BI__builtin_strcmp &&
         BuiltinOp != Builtin::BI__builtin_wcscmp) {
       APSInt N;
       if (!EvaluateInteger(E->getArg(2), N, Info))
         return false;
-      MaxLength = N.getExtValue();
+      MaxLength = N.getZExtValue();
     }
 
     // Empty substrings compare equal by definition.
     if (MaxLength == 0u)
       return Success(0, E);
 
     if (!String1.checkNullPointerForFoldAccess(Info, E, AK_Read) ||
         !String2.checkNullPointerForFoldAccess(Info, E, AK_Read) ||

clang/test/SemaCXX/constexpr-string.cpp

namespace StrcmpEtc {
   static_assert(__builtin_strncmp("abaa", "abba", 2) == 0);
   static_assert(__builtin_strncmp("abaa", "abba", 1) == 0);
   static_assert(__builtin_strncmp("abaa", "abba", 0) == 0);
   static_assert(__builtin_strncmp(0, 0, 0) == 0);
   static_assert(__builtin_strncmp("abab\0banana", "abab\0canada", 100) == 0);
 
   static_assert(__builtin_strncmp(kFoobar, kFoobazfoobar, 6) == -1);
   static_assert(__builtin_strncmp(kFoobar, kFoobazfoobar, 7) == -1); // FIXME: Should we reject this?
+  static_assert(__builtin_strncmp(kFoobar, kFoobazfoobar, (size_t)-1) == -1); // Check that this does not assert
   static_assert(__builtin_strncmp(kFoobar, kFoobazfoobar + 6, 6) == 0);
   static_assert(__builtin_strncmp(kFoobar, kFoobazfoobar + 6, 7) == 0); // expected-error {{not an integral constant}} expected-note {{dereferenced one-past-the-end}}
 
   static_assert(__builtin_memcmp("abaa", "abba", 3) == -1);
   static_assert(__builtin_memcmp("abaa", "abba", 2) == 0);
   static_assert(__builtin_memcmp("a\203", "a", 2) == 1);
   static_assert(__builtin_memcmp("a\203", "a\003", 2) == 1);
   static_assert(__builtin_memcmp(0, 0, 0) == 0);
namespace StrchrEtc {
   static_assert(__builtin_strchr(kStr, '\xff' + 256) == nullptr);
   static_assert(__builtin_strchr(kStr, '\xff' - 256) == nullptr);
   static_assert(__builtin_strchr(kFoo, 'o') == kFoo + 1);
   static_assert(__builtin_strchr(kFoo, 'x') == nullptr); // expected-error {{not an integral constant}} expected-note {{dereferenced one-past-the-end}}
   static_assert(__builtin_strchr(nullptr, 'x') == nullptr); // expected-error {{not an integral constant}} expected-note {{dereferenced null}}
 
   static_assert(__builtin_memchr(kStr, 'a', 0) == nullptr);
   static_assert(__builtin_memchr(kStr, 'a', 1) == kStr);
+  static_assert(__builtin_memchr(kStr, 'a', (size_t)-1) == kStr); // Check that this does not assert
   static_assert(__builtin_memchr(kStr, '\0', 5) == nullptr);
   static_assert(__builtin_memchr(kStr, '\0', 6) == kStr + 5);
   static_assert(__builtin_memchr(kStr, '\xff', 8) == kStr + 4);
   static_assert(__builtin_memchr(kStr, '\xff' + 256, 8) == kStr + 4);
   static_assert(__builtin_memchr(kStr, '\xff' - 256, 8) == kStr + 4);
   static_assert(__builtin_memchr(kFoo, 'x', 3) == nullptr);
   static_assert(__builtin_memchr(kFoo, 'x', 4) == nullptr); // expected-error {{not an integral constant}} expected-note {{dereferenced one-past-the-end}}
   static_assert(__builtin_memchr(nullptr, 'x', 3) == nullptr); // expected-error {{not an integral constant}} expected-note {{dereferenced null}}