[Clang][C++23] Implement P1774R8: Portable assumptions (#81014)

This implements the C++23 `[[assume]]` attribute.

Assumption information is lowered to a call to `@llvm.assume`, unless the expression has side-effects, in which case it is discarded and a warning is issued to tell the user that the assumption doesn’t do anything. A failed assumption at compile time is an error (unless we are in `MSVCCompat` mode, in which case we don’t check assumptions at compile time).

Due to performance regressions in LLVM, assumptions can be disabled with the `-fno-assumptions` flag. With it, assumptions will still be parsed and checked, but no calls to `@llvm.assume` will be emitted and assumptions will not be checked at compile time.

1 files changed, 23 insertions, 0 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 4a7c775..726415c 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -5582,6 +5582,29 @@ static EvalStmtResult EvaluateStmt(StmtResult &Result, EvalInfo &Info,
     MSConstexprContextRAII ConstexprContext(
         *Info.CurrentCall, hasSpecificAttr<MSConstexprAttr>(AS->getAttrs()) &&
                                isa<ReturnStmt>(SS));
+
+    auto LO = Info.getCtx().getLangOpts();
+    if (LO.CXXAssumptions && !LO.MSVCCompat) {
+      for (auto *Attr : AS->getAttrs()) {
+        auto *AA = dyn_cast<CXXAssumeAttr>(Attr);
+        if (!AA)
+          continue;
+
+        auto *Assumption = AA->getAssumption();
+        if (Assumption->isValueDependent())
+          return ESR_Failed;
+
+        bool Value;
+        if (!EvaluateAsBooleanCondition(Assumption, Value, Info))
+          return ESR_Failed;
+        if (!Value) {
+          Info.CCEDiag(Assumption->getExprLoc(),
+                       diag::note_constexpr_assumption_failed);
+          return ESR_Failed;
+        }
+      }
+    }
+
     return EvaluateStmt(Result, Info, SS, Case);
   }