Clean up denormal handling with -ffp-model, -ffast-math, etc.

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 +1452,8 @@ 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"
+  "denormal_fp_math", "IEEE", "IEEE", "target-dependent"
+  "denormal_fp32_math", "IEEE","IEEE", "target-dependent"
   "support_math_errno", "on", "on", "off"
   "no_honor_nans", "off", "off", "on"
   "no_honor_infinities", "off", "off", "on"

clang/lib/Driver/ToolChain.cpp

@@ -1319,11 +1319,17 @@ bool ToolChain::isFastMathRuntimeAvailable(const ArgList &Args,
     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_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

@@ -2924,6 +2924,10 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
       if (Val.equals("fast")) {
         FPModel = Val;
         applyFastMath();
+        // The target-specific getDefaultDenormalModeForType handler should
+        // account for -ffp-model=fast and choose its behavior
+        DenormalFPMath = DefaultDenormalFPMath;
+        DenormalFP32Math = DefaultDenormalFP32Math;
       } else if (Val.equals("precise")) {
         optID = options::OPT_ffp_contract;
         FPModel = Val;

clang/test/Driver/linux-ld.c

@@ -1417,6 +1417,9 @@
 // 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