[flang-commits] [flang] [draft][flang] Query backend support for quad-precision compilation decision (PR #182230)

[Tarun Prabhu via flang-commits]

================
@@ -58,17 +62,50 @@ namespace Fortran::tools {
     break;
   }
 
-  // Check for kind=16 support. See flang/runtime/Float128Math/math-entries.h.
-  // TODO: Take this from TargetInfo::getLongDoubleFormat for cross compilation.
-#ifdef FLANG_RUNTIME_F128_MATH_LIB
-  constexpr bool f128Support = true; // use libquadmath wrappers
-#elif HAS_LDBL128
-  constexpr bool f128Support = true; // use libm wrappers
-#else
-  constexpr bool f128Support = false;
-#endif
-
-  if constexpr (!f128Support) {
+  bool f128Support = false;
+  llvm::LLVMContext ctx;
+  std::unique_ptr<llvm::Module> dummyModule =
+      std::make_unique<llvm::Module>("quad-test", ctx);
+  dummyModule->setTargetTriple(targetMachine.getTargetTriple());
+  dummyModule->setDataLayout(targetMachine.createDataLayout());
+
+  llvm::FunctionType *dummyFTy =
+      llvm::FunctionType::get(llvm::Type::getVoidTy(ctx), false);
+  llvm::Function *dummyF = llvm::Function::Create(dummyFTy,
+      llvm::GlobalValue::ExternalLinkage, "quad-test", dummyModule.get());
+
+  const llvm::TargetLowering *dummyTLI =
+      targetMachine.getSubtargetImpl(*dummyF)->getTargetLowering();
+
+  if (dummyTLI) {
+    llvm::EVT fp128EVT = llvm::EVT::getEVT(llvm::Type::getFP128Ty(ctx));
+
+    // Query for fp128 backend support. Based on this, determine whether
+    // compilation is possible on the frontend.
+    bool isLegal = dummyTLI->isTypeLegal(fp128EVT);
+
+    // We might also be able to determine fp128 backend support based on the
+    // LegalizeAction value. This is likely when the value is "Legal" or
+    // "LibCall". See
+    // https://llvm.org/doxygen/TargetLowering_8h_source.html#l00202.
+    llvm::TargetLowering::LegalizeAction LA =
+        dummyTLI->getOperationAction(llvm::ISD::FADD, fp128EVT);
+
+    // We might also be able to determine fp128 backend support based on the
+    // LegalizeTypeAction value. This is likely when the value is "TypeLegal".
+    // See https://llvm.org/doxygen/TargetLowering_8h_source.html#l00212.
+    llvm::TargetLowering::LegalizeTypeAction LTA =
+        dummyTLI->getTypeConversion(ctx, llvm::MVT::f128).first;
+
+    if (isLegal &&
+        (LA == llvm::TargetLowering::Legal ||
----------------
tarunprabhu wrote:

> I thought that quad-precision would be supported if libcall could be used, even without native support, so I also checked the legalize action.

But this is checking `LegalizeAction` if `isLegal` is already true. Was this intended to be `isLegal ||` ? 

For an initial implementation, it may be ok limit ourselves to platforms on which quad-precision floating point is natively supported.

https://github.com/llvm/llvm-project/pull/182230