[clang] [Clang] Allow __builtin_fma/fmaf/fmal to be used in a constant expression (PR #158048)

[Chaitanya Koparkar via cfe-commits]

https://github.com/ckoparkar created https://github.com/llvm/llvm-project/pull/158048

Fixes #154747.

>From 7665f0d152250b327b93a8aedc2ed9410e22277c Mon Sep 17 00:00:00 2001
From: Chaitanya Koparkar <ckoparkar at gmail.com>
Date: Thu, 11 Sep 2025 07:24:02 -0400
Subject: [PATCH] [Clang] Allow __builtin_fma/fmaf/fmal to be used in a
 constant expression

---
 clang/include/clang/Basic/Builtins.td    |  2 +-
 clang/lib/AST/ByteCode/InterpBuiltin.cpp |  8 ++++++++
 clang/lib/AST/ExprConstant.cpp           | 12 ++++++++----
 3 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/clang/include/clang/Basic/Builtins.td b/clang/include/clang/Basic/Builtins.td
index 27639f06529cb..a22a6e68db83b 100644
--- a/clang/include/clang/Basic/Builtins.td
+++ b/clang/include/clang/Basic/Builtins.td
@@ -199,7 +199,7 @@ def FloorF16F128 : Builtin, F16F128MathTemplate {
 
 def FmaF16F128 : Builtin, F16F128MathTemplate {
   let Spellings = ["__builtin_fma"];
-  let Attributes = [FunctionWithBuiltinPrefix, NoThrow, ConstIgnoringErrnoAndExceptions];
+  let Attributes = [FunctionWithBuiltinPrefix, NoThrow, ConstIgnoringErrnoAndExceptions, Constexpr];
   let Prototype = "T(T, T, T)";
 }
 
diff --git a/clang/lib/AST/ByteCode/InterpBuiltin.cpp b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
index a0dcdace854b9..55baa4891bb32 100644
--- a/clang/lib/AST/ByteCode/InterpBuiltin.cpp
+++ b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
@@ -3411,6 +3411,14 @@ bool InterpretBuiltin(InterpState &S, CodePtr OpPC, const CallExpr *Call,
   case clang::X86::BI__builtin_ia32_pmuludq512:
     return interp__builtin_ia32_pmul(S, OpPC, Call, BuiltinID);
 
+  case Builtin::BIfma:
+  case Builtin::BIfmal:
+  case Builtin::BIfmaf:
+  case Builtin::BI__builtin_fma:
+  case Builtin::BI__builtin_fmal:
+  case Builtin::BI__builtin_fmaf:
+  case Builtin::BI__builtin_fmaf16:
+  case Builtin::BI__builtin_fmaf128:
   case Builtin::BI__builtin_elementwise_fma:
     return interp__builtin_elementwise_triop_fp(
         S, OpPC, Call,
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index ca930737474df..dbff682714342 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -16346,11 +16346,15 @@ bool FloatExprEvaluator::VisitCallExpr(const CallExpr *E) {
     return true;
   }
 
+  case Builtin::BIfma:
+  case Builtin::BIfmal:
+  case Builtin::BIfmaf:
+  case Builtin::BI__builtin_fma:
+  case Builtin::BI__builtin_fmal:
+  case Builtin::BI__builtin_fmaf:
+  case Builtin::BI__builtin_fmaf16:
+  case Builtin::BI__builtin_fmaf128:
   case Builtin::BI__builtin_elementwise_fma: {
-    if (!E->getArg(0)->isPRValue() || !E->getArg(1)->isPRValue() ||
-        !E->getArg(2)->isPRValue()) {
-      return false;
-    }
     APFloat SourceY(0.), SourceZ(0.);
     if (!EvaluateFloat(E->getArg(0), Result, Info) ||
         !EvaluateFloat(E->getArg(1), SourceY, Info) ||