[Driver] Clean up denormal handling with fast-math-related options (#89477)

This change cleans up the clang driver handling of umbrella options like
-ffast-math, -funsafe-math-optimizations, and -ffp-model, and aligns the
behavior of -ffp-model=fast with -ffast-math with regard to the linking
of crtfastmath.o.

We agreed in a previous review that the fast-math options should not
attempt to change the -fdenormal-fp-math option, which is inherently
target-specific.

The clang user's manual claims that -ffp-model=fast "behaves identically
to specifying both -ffast-math and -ffp-contract=fast." Since
-ffast-math causes crtfastmath.o to be linked if it is available, that should
also happen with -ffp-model=fast.

I am going to be proposing further changes to -ffp-model=fast,
decoupling it from -ffast-math and introducing a new
-ffp-model=aggressive that matches the current behavior, but I wanted
to solidfy the current behavior before I do that.

Author: Andy Kaylor

clang/docs/UsersManual.rst

@@ -1452,8 +1452,6 @@ floating point semantic models: precise (the default), strict, and fast.
   "fenv_access", "off", "on", "off"
   "rounding_mode", "tonearest", "dynamic", "tonearest"
   "contract", "on", "off", "fast"
-  "denormal_fp_math", "IEEE", "IEEE", "IEEE"
-  "denormal_fp32_math", "IEEE","IEEE", "IEEE"
   "support_math_errno", "on", "on", "off"
   "no_honor_nans", "off", "off", "on"
   "no_honor_infinities", "off", "off", "on"
@@ -1462,6 +1460,14 @@ floating point semantic models: precise (the default), strict, and fast.
   "allow_approximate_fns", "off", "off", "on"
   "allow_reassociation", "off", "off", "on"
 
+The ``-ffp-model`` option does not modify the ``fdenormal-fp-math``
+setting, but it does have an impact on whether ``crtfastmath.o`` is
+linked. Because linking ``crtfastmath.o`` has a global effect on the
+program, and because the global denormal handling can be changed in
+other ways, the state of ``fdenormal-fp-math`` handling cannot
+be assumed in any function based on fp-model. See :ref:`crtfastmath.o`
+for more details.
+
 .. option:: -ffast-math
 
    Enable fast-math mode.  This option lets the
@@ -1537,7 +1543,6 @@ floating point semantic models: precise (the default), strict, and fast.
    Also, this option resets following options to their target-dependent defaults.
 
    * ``-f[no-]math-errno``
-   * ``-fdenormal-fp-math=<value>``
 
    There is ambiguity about how ``-ffp-contract``, ``-ffast-math``,
    and ``-fno-fast-math`` behave when combined. To keep the value of
@@ -1560,8 +1565,7 @@ floating point semantic models: precise (the default), strict, and fast.
      ``-ffp-contract`` setting is determined by the default value of
      ``-ffp-contract``.
 
-   Note: ``-fno-fast-math`` implies ``-fdenormal-fp-math=ieee``.
-   ``-fno-fast-math`` causes ``crtfastmath.o`` to not be linked with code
+   Note: ``-fno-fast-math`` causes ``crtfastmath.o`` to not be linked with code
    unless ``-mdaz-ftz`` is present.
 
 .. option:: -fdenormal-fp-math=<value>
@@ -1694,7 +1698,6 @@ floating point semantic models: precise (the default), strict, and fast.
    * ``-fsigned-zeros``
    * ``-ftrapping-math``
    * ``-ffp-contract=on``
-   * ``-fdenormal-fp-math=ieee``
 
    There is ambiguity about how ``-ffp-contract``,
    ``-funsafe-math-optimizations``, and ``-fno-unsafe-math-optimizations``

clang/lib/Driver/ToolChain.cpp

@@ -1316,14 +1316,19 @@ bool ToolChain::isFastMathRuntimeAvailable(const ArgList &Args,
   // (to keep the linker options consistent with gcc and clang itself).
   if (Default && !isOptimizationLevelFast(Args)) {
     // Check if -ffast-math or -funsafe-math.
-    Arg *A =
-      Args.getLastArg(options::OPT_ffast_math, options::OPT_fno_fast_math,
-                      options::OPT_funsafe_math_optimizations,
-                      options::OPT_fno_unsafe_math_optimizations);
+    Arg *A = Args.getLastArg(
+        options::OPT_ffast_math, options::OPT_fno_fast_math,
+        options::OPT_funsafe_math_optimizations,
+        options::OPT_fno_unsafe_math_optimizations, options::OPT_ffp_model_EQ);
 
     if (!A || A->getOption().getID() == options::OPT_fno_fast_math ||
         A->getOption().getID() == options::OPT_fno_unsafe_math_optimizations)
       Default = false;
+    if (A && A->getOption().getID() == options::OPT_ffp_model_EQ) {
+      StringRef Model = A->getValue();
+      if (Model != "fast")
+        Default = false;
+    }
   }
 
   // Whatever decision came as a result of the above implicit settings, either

clang/lib/Driver/ToolChains/Clang.cpp

@@ -2743,13 +2743,11 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
   StringRef FPExceptionBehavior = "";
   // -ffp-eval-method options: double, extended, source
   StringRef FPEvalMethod = "";
-  const llvm::DenormalMode DefaultDenormalFPMath =
+  llvm::DenormalMode DenormalFPMath =
       TC.getDefaultDenormalModeForType(Args, JA);
-  const llvm::DenormalMode DefaultDenormalFP32Math =
+  llvm::DenormalMode DenormalFP32Math =
       TC.getDefaultDenormalModeForType(Args, JA, &llvm::APFloat::IEEEsingle());
 
-  llvm::DenormalMode DenormalFPMath = DefaultDenormalFPMath;
-  llvm::DenormalMode DenormalFP32Math = DefaultDenormalFP32Math;
   // CUDA and HIP don't rely on the frontend to pass an ffp-contract option.
   // If one wasn't given by the user, don't pass it here.
   StringRef FPContract;
@@ -2899,11 +2897,6 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
       SignedZeros = true;
       // -fno_fast_math restores default denormal and fpcontract handling
       FPContract = "on";
-      DenormalFPMath = llvm::DenormalMode::getIEEE();
-
-      // FIXME: The target may have picked a non-IEEE default mode here based on
-      // -cl-denorms-are-zero. Should the target consider -fp-model interaction?
-      DenormalFP32Math = llvm::DenormalMode::getIEEE();
 
       StringRef Val = A->getValue();
       if (OFastEnabled && !Val.equals("fast")) {
@@ -3122,9 +3115,6 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
       TrappingMath = true;
       FPExceptionBehavior = "strict";
 
-      // The target may have opted to flush by default, so force IEEE.
-      DenormalFPMath = llvm::DenormalMode::getIEEE();
-      DenormalFP32Math = llvm::DenormalMode::getIEEE();
       if (!JA.isDeviceOffloading(Action::OFK_Cuda) &&
           !JA.isOffloading(Action::OFK_HIP)) {
         if (LastSeenFfpContractOption != "") {
@@ -3154,9 +3144,7 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
       ReciprocalMath = false;
       ApproxFunc = false;
       SignedZeros = true;
-      // -fno_fast_math restores default denormal and fpcontract handling
-      DenormalFPMath = DefaultDenormalFPMath;
-      DenormalFP32Math = llvm::DenormalMode::getIEEE();
+      // -fno_fast_math restores default fpcontract handling
       if (!JA.isDeviceOffloading(Action::OFK_Cuda) &&
           !JA.isOffloading(Action::OFK_HIP)) {
         if (LastSeenFfpContractOption != "") {
@@ -3171,8 +3159,6 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
       // subsequent options conflict then emit warning diagnostic.
       if (HonorINFs && HonorNaNs && !AssociativeMath && !ReciprocalMath &&
           SignedZeros && TrappingMath && RoundingFPMath && !ApproxFunc &&
-          DenormalFPMath == llvm::DenormalMode::getIEEE() &&
-          DenormalFP32Math == llvm::DenormalMode::getIEEE() &&
           FPContract.equals("off"))
         // OK: Current Arg doesn't conflict with -ffp-model=strict
         ;

clang/test/Driver/fp-model.c

@@ -64,7 +64,8 @@
 
 // RUN: %clang -### -ffp-model=strict -fdenormal-fp-math=preserve-sign,preserve-sign -c %s 2>&1 \
 // RUN:   | FileCheck --check-prefix=WARN10 %s
-// WARN10: warning: overriding '-ffp-model=strict' option with '-fdenormal-fp-math=preserve-sign,preserve-sign' [-Woverriding-option]
+// WARN10: "-cc1"
+// WARN10-NOT: warning: overriding '-ffp-model=strict' option with '-fdenormal-fp-math=preserve-sign,preserve-sign' [-Woverriding-option]
 
 // RUN: %clang -### -ffp-model=fast -ffp-model=strict -c %s 2>&1 | FileCheck \
 // RUN:   --check-prefix=WARN11 %s
@@ -128,6 +129,7 @@
 // RUN:   | FileCheck --check-prefix=CHECK-NO-EXCEPT %s
 // RUN: %clang -### -nostdinc -ffp-model=strict -Ofast -c %s 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-NO-EXCEPT %s
+// CHECK-NO-EXCEPT: "-cc1"
 // CHECK-NO-EXCEPT-NOT: "-ffp-exception-behavior=strict"
 
 // RUN: %clang -### -nostdinc -ffp-exception-behavior=strict -c %s 2>&1 \

clang/test/Driver/linux-ld.c

@@ -1406,6 +1406,9 @@
 // RUN: %clang --target=x86_64-unknown-linux -no-pie -### %s -funsafe-math-optimizations\
 // RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-CRTFASTMATH %s
+// RUN: %clang --target=x86_64-unknown-linux -no-pie -### %s -ffp-model=fast \
+// RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-CRTFASTMATH %s
 // RUN: %clang --target=x86_64-unknown-linux -no-pie -### %s -Ofast\
 // RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-CRTFASTMATH %s

clang/test/Driver/solaris-ld.c

@@ -193,6 +193,9 @@
 // RUN: %clang --target=sparc-sun-solaris2.11 -### %s -ffast-math \
 // RUN:        --sysroot=%S/Inputs/solaris_sparc_tree 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-CRTFASTMATH-SPARC32 %s
+// RUN: %clang --target=sparc-sun-solaris2.11 -### %s -ffp-model=fast \
+// RUN:        --sysroot=%S/Inputs/solaris_sparc_tree 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-CRTFASTMATH-SPARC32 %s
 // CHECK-CRTFASTMATH-SPARC32: "-isysroot" "[[SYSROOT:[^"]+]]"
 // CHECK-CRTFASTMATH-SPARC32: "[[SYSROOT]]/usr/gcc/4.8/lib/gcc/sparc-sun-solaris2.11/4.8.2{{/|\\\\}}crtfastmath.o"
 // CHECK-NOCRTFASTMATH-SPARC32-NOT: crtfastmath.o