[CodeGen] use saturating FP casts when compiling with "no-strict-floa…

…t-cast-overflow"

We got an unintended consequence of the optimizer getting smarter when
compiling in a non-standard mode, and there's no good way to inhibit
those optimizations at a later stage. The test is based on an example
linked from D92270.

We allow the "no-strict-float-cast-overflow" exception to normal C
cast rules to preserve legacy code that does not expect overflowing
casts from FP to int to produce UB. See D46236 for details.

Differential Revision: https://reviews.llvm.org/D115804

clang/lib/CodeGen/CGExprScalar.cpp

@@ -1240,7 +1240,18 @@ Value *ScalarExprEmitter::EmitScalarCast(Value *Src, QualType SrcType,
 
   if (isa<llvm::IntegerType>(DstElementTy)) {
     assert(SrcElementTy->isFloatingPointTy() && "Unknown real conversion");
-    if (DstElementType->isSignedIntegerOrEnumerationType())
+    bool IsSigned = DstElementType->isSignedIntegerOrEnumerationType();
+
+    // If we can't recognize overflow as undefined behavior, assume that
+    // overflow saturates. This protects against normal optimizations if we are
+    // compiling with non-standard FP semantics.
+    if (!CGF.CGM.getCodeGenOpts().StrictFloatCastOverflow) {
+      llvm::Intrinsic::ID IID =
+          IsSigned ? llvm::Intrinsic::fptosi_sat : llvm::Intrinsic::fptoui_sat;
+      return Builder.CreateCall(CGF.CGM.getIntrinsic(IID, {DstTy, SrcTy}), Src);
+    }
+
+    if (IsSigned)
       return Builder.CreateFPToSI(Src, DstTy, "conv");
     return Builder.CreateFPToUI(Src, DstTy, "conv");
   }

clang/test/CodeGen/no-junk-ftrunc.c

@@ -1,14 +1,22 @@
 // RUN: %clang_cc1 -S -fno-strict-float-cast-overflow %s -emit-llvm -o - | FileCheck %s --check-prefix=NOSTRICT
+
+// When compiling with non-standard semantics, use intrinsics to inhibit the optimizer.
+
 // NOSTRICT-LABEL: main
+// NOSTRICT: call i32 @llvm.fptosi.sat.i32.f64
+// NOSTRICT: call i32 @llvm.fptoui.sat.i32.f64
 // NOSTRICT: attributes #0 = {{.*}}"strict-float-cast-overflow"="false"{{.*}}
 
 // The workaround attribute is not applied by default.
 
 // RUN: %clang_cc1 -S %s -emit-llvm -o - | FileCheck %s --check-prefix=STRICT
 // STRICT-LABEL: main
+// STRICT: = fptosi
+// STRICT: = fptoui
 // STRICT-NOT: strict-float-cast-overflow
 
+
 int main() {
-  return 0;
+  double d = 1e20;
+  return (int)d != 1e20 && (unsigned)d != 1e20;
 }
-