Align -ffp-model=fast denormal handling with -ffast-math

This is an attempt to better align the denormal handling of
-ffp-model=fast with the behavior of -ffast-math. The clang user's
manual claims that -ffp-model=fast "behaves identically to specifying
both -ffast-math and -ffp-contract=fast." That isn't entirely correct.
One difference is that -ffast-math causes crtfastmath.o to be linked if
it is available, and passes -fdenormal-fp-math=PreserveSign as a -cc1
option when crtfastmath.o is available.

I'm not sure the current behavior is reasonable and consistent for all
tool chains, but I'm not trying to fix that here. This is just trying to
make an incremental improvement.

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

@@ -1314,11 +1314,17 @@ 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)
       return false;
+    if (A && A->getOption().getID() == options::OPT_ffp_model_EQ) {
+      StringRef Model = A->getValue();
+      if (!Model.equals("fast"))
+        return false;
+    }
   }
   // If crtfastmath.o exists add it to the arguments.
   Path = GetFilePath("crtfastmath.o");

clang/lib/Driver/ToolChains/Clang.cpp

@@ -2942,6 +2942,10 @@ 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/default-denormal-fp-math.c

@@ -5,12 +5,15 @@
 
 // crtfastmath enables ftz and daz
 // RUN: %clang -### -target x86_64-unknown-linux-gnu -ffast-math --sysroot=%S/Inputs/basic_linux_tree -c %s -v 2>&1 | FileCheck -check-prefix=CHECK-PRESERVESIGN %s
+// RUN: %clang -### -target x86_64-unknown-linux-gnu -ffp-model=fast --sysroot=%S/Inputs/basic_linux_tree -c %s -v 2>&1 | FileCheck -check-prefix=CHECK-PRESERVESIGN %s
 
 // crt not linked in with nostartfiles
 // RUN: %clang -### -target x86_64-unknown-linux-gnu -ffast-math -nostartfiles --sysroot=%S/Inputs/basic_linux_tree -c %s -v 2>&1 | FileCheck -check-prefix=CHECK-IEEE %s
+// RUN: %clang -### -target x86_64-unknown-linux-gnu -ffp-model=fast -nostartfiles --sysroot=%S/Inputs/basic_linux_tree -c %s -v 2>&1 | FileCheck -check-prefix=CHECK-IEEE %s
 
 // If there's no crtfastmath, don't assume ftz/daz
 // RUN: %clang -### -target x86_64-unknown-linux-gnu -ffast-math --sysroot=/dev/null -c %s -v 2>&1 | FileCheck -check-prefix=CHECK-IEEE %s
+// RUN: %clang -### -target x86_64-unknown-linux-gnu -ffp-model=fast --sysroot=/dev/null -c %s -v 2>&1 | FileCheck -check-prefix=CHECK-IEEE %s
 
 // RUN: %clang -### -target x86_64-scei-ps4 -c %s -v 2>&1 | FileCheck -check-prefix=CHECK-PRESERVESIGN %s
 

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