[Driver, CodeGen] add options to enable/disable an FP cast optimization

As discussed in the post-commit thread for:
rL330437 ( http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20180423/545906.html )

We need a way to opt-out of a float-to-int-to-float cast optimization because too much 
existing code relies on the platform-specific undefined result of those casts when the 
float-to-int overflows.

The LLVM changes associated with adding this function attribute are here:
rL330947
rL330950
rL330951

Also as suggested, I changed the LLVM doc to mention the specific sanitizer flag that 
catches this problem:
rL330958

Differential Revision: https://reviews.llvm.org/D46135

llvm-svn: 331041

3 files changed, 10 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index eed9e2f5c40..da2ff14b598 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -1727,6 +1727,9 @@ void CodeGenModule::ConstructDefaultFnAttrList(StringRef Name, bool HasOptnone,
     FuncAttrs.addAttribute("no-trapping-math",
                            llvm::toStringRef(CodeGenOpts.NoTrappingMath));
 
+    if (CodeGenOpts.FPCastOverflowWorkaround)
+      FuncAttrs.addAttribute("fp-cast-overflow-workaround", "true");
+
     // TODO: Are these all needed?
     // unsafe/inf/nan/nsz are handled by instruction-level FastMathFlags.
     FuncAttrs.addAttribute("no-infs-fp-math",
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 5751ceac710..a0a5a5cc715 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -2241,6 +2241,11 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
     CmdArgs.push_back("-mfpmath");
     CmdArgs.push_back(A->getValue());
   }
+
+  // Disable a codegen optimization for floating-point casts.
+  if (Args.hasFlag(options::OPT_ffp_cast_overflow_workaround,
+                   options::OPT_fno_fp_cast_overflow_workaround, false))
+    CmdArgs.push_back("-ffp-cast-overflow-workaround");
 }
 
 static void RenderAnalyzerOptions(const ArgList &Args, ArgStringList &CmdArgs,
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index f2f22ee2c39..9ef4ad77558 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -699,6 +699,8 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
   Opts.Reciprocals = Args.getAllArgValues(OPT_mrecip_EQ);
   Opts.ReciprocalMath = Args.hasArg(OPT_freciprocal_math);
   Opts.NoTrappingMath = Args.hasArg(OPT_fno_trapping_math);
+  Opts.FPCastOverflowWorkaround = Args.hasArg(OPT_ffp_cast_overflow_workaround);
+
   Opts.NoZeroInitializedInBSS = Args.hasArg(OPT_mno_zero_initialized_in_bss);
   Opts.NumRegisterParameters = getLastArgIntValue(Args, OPT_mregparm, 0, Diags);
   Opts.NoExecStack = Args.hasArg(OPT_mno_exec_stack);