[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@331041 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/docs/UsersManual.rst b/docs/UsersManual.rst
index 3653d0088eb53601965a34cf359434e538528e5e..7425d2ad974492e399902675ae98ba4891bc9067 100644 (file)
--- a/docs/UsersManual.rst
+++ b/docs/UsersManual.rst
@@ -1255,6 +1255,16 @@ are listed below.
    flushed-to-zero number is preserved in the sign of 0, denormals are
    flushed to positive zero, respectively.
 
+.. option:: -f[no-]fp-cast-overflow-workaround
+
+   Enable a workaround for code that casts floating-point values to 
+   integers and back to floating-point. If the floating-point value 
+   is not representable in the intermediate integer type, the code is
+   incorrect according to the language standard. This flag will attempt 
+   to generate code as if the result of an overflowing conversion matches
+   the overflowing behavior of a target's native float-to-int conversion
+   instructions.
+
 .. option:: -fwhole-program-vtables
 
    Enable whole-program vtable optimizations, such as single-implementation

diff --git a/include/clang/Driver/Options.td b/include/clang/Driver/Options.td
index e3ced42154c2e04f9973c45bd8953ce03c491466..f18a3b17463020bba6d0701a162b2874cc8e1d5a 100644 (file)
--- a/include/clang/Driver/Options.td
+++ b/include/clang/Driver/Options.td
@@ -1029,6 +1029,11 @@ def ffp_contract : Joined<["-"], "ffp-contract=">, Group<f_Group>,
   Flags<[CC1Option]>, HelpText<"Form fused FP ops (e.g. FMAs): fast (everywhere)"
   " | on (according to FP_CONTRACT pragma, default) | off (never fuse)">, Values<"fast,on,off">;
 
+def ffp_cast_overflow_workaround : Flag<["-"],
+  "ffp-cast-overflow-workaround">, Group<f_Group>, Flags<[CC1Option]>;
+def fno_fp_cast_overflow_workaround : Flag<["-"],
+  "fno-fp-cast-overflow-workaround">, Group<f_Group>, Flags<[CC1Option]>;
+
 def ffor_scope : Flag<["-"], "ffor-scope">, Group<f_Group>;
 def fno_for_scope : Flag<["-"], "fno-for-scope">, Group<f_Group>;
 

diff --git a/include/clang/Frontend/CodeGenOptions.def b/include/clang/Frontend/CodeGenOptions.def
index 1684dbdb5576d062ab8e359b5c78022901c2f73f..1747aa7287fc8e5bb689e45d52e75190cb50df08 100644 (file)
--- a/include/clang/Frontend/CodeGenOptions.def
+++ b/include/clang/Frontend/CodeGenOptions.def
@@ -136,6 +136,12 @@ CODEGENOPT(NoTrappingMath    , 1, 0) ///< Set when -fno-trapping-math is enabled
 CODEGENOPT(NoNaNsFPMath      , 1, 0) ///< Assume FP arguments, results not NaN.
 CODEGENOPT(FlushDenorm       , 1, 0) ///< Allow FP denorm numbers to be flushed to zero
 CODEGENOPT(CorrectlyRoundedDivSqrt, 1, 0) ///< -cl-fp32-correctly-rounded-divide-sqrt
+
+/// Disable a float-to-int-to-float cast optimization. This attempts to generate
+/// code as if the result of an overflowing conversion matches the overflowing
+/// behavior of a target's native float-to-int conversion instructions.
+CODEGENOPT(FPCastOverflowWorkaround, 1, 0)
+
 CODEGENOPT(UniformWGSize     , 1, 0) ///< -cl-uniform-work-group-size
 CODEGENOPT(NoZeroInitializedInBSS , 1, 0) ///< -fno-zero-initialized-in-bss.
 /// \brief Method of Objective-C dispatch to use.

diff --git a/lib/CodeGen/CGCall.cpp b/lib/CodeGen/CGCall.cpp
index eed9e2f5c40d07c06c6a0428ff7781d9156539e7..da2ff14b598e90834a7f4f6f36ebdcd0eea3ac02 100644 (file)
--- a/lib/CodeGen/CGCall.cpp
+++ b/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/lib/Driver/ToolChains/Clang.cpp b/lib/Driver/ToolChains/Clang.cpp
index 5751ceac7107c2c8a85b359e49d5051a21d45139..a0a5a5cc71555ac0f27e46318c84c6213be9b7da 100644 (file)
--- a/lib/Driver/ToolChains/Clang.cpp
+++ b/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/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index f2f22ee2c39b34f6e227ea0ad360617d84bfcaba..9ef4ad7755844a1ed5cf6046b480dcbb4cef3123 100644 (file)
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/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);

diff --git a/test/CodeGen/no-junk-ftrunc.c b/test/CodeGen/no-junk-ftrunc.c
new file mode 100644 (file)
index 0000000..ff3f640
--- /dev/null
+++ b/test/CodeGen/no-junk-ftrunc.c
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -S -ffp-cast-overflow-workaround %s -emit-llvm -o - | FileCheck %s
+// CHECK-LABEL: main
+// CHECK: attributes #0 = {{.*}}"fp-cast-overflow-workaround"="true"{{.*}}
+
+// The workaround attribute is not applied by default.
+
+// RUN: %clang_cc1 -S %s -emit-llvm -o - | FileCheck %s --check-prefix=DEFAULT
+// DEFAULT-LABEL: main
+// DEFAULT-NOT: fp-cast-overflow-workaround
+
+int main() {
+  return 0;
+}
+

diff --git a/test/Driver/fast-math.c b/test/Driver/fast-math.c
index 7bb057fcf7d6bc930e45d282e64985a869861f27..e1e07f37742a3f17395c4a92058e289b01034c90 100644 (file)
--- a/test/Driver/fast-math.c
+++ b/test/Driver/fast-math.c
@@ -287,3 +287,27 @@
 // RUN: %clang -### -ftrapping-math -fno-trapping-math -c %s 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-NO-TRAPPING-MATH %s
 // CHECK-NO-TRAPPING-MATH: "-fno-trapping-math"
+
+// This isn't fast-math, but the option is handled in the same place as other FP params.
+// Last option wins, and the flag is *not* passed by default. 
+
+// RUN: %clang -### -ffp-cast-overflow-workaround -c %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-FPOV-WORKAROUND %s
+// CHECK-FPOV-WORKAROUND: "-cc1"
+// CHECK-FPOV-WORKAROUND: "-ffp-cast-overflow-workaround"
+
+// RUN: %clang -### -c %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-FPOV-WORKAROUND-DEFAULT %s
+// CHECK-FPOV-WORKAROUND-DEFAULT: "-cc1"
+// CHECK-FPOV-WORKAROUND-DEFAULT-NOT: "-ffp-cast-overflow-workaround"
+
+// RUN: %clang -### -fno-fp-cast-overflow-workaround -c %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-NO-FPOV-WORKAROUND %s
+// CHECK-NO-FPOV-WORKAROUND: "-cc1"
+// CHECK-NO-FPOV-WORKAROUND-NOT: "-ffp-cast-overflow-workaround"
+
+// RUN: %clang -### -ffp-cast-overflow-workaround -fno-fp-cast-overflow-workaround -c %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-NO-FPOV-WORKAROUND-OVERRIDE %s
+// CHECK-NO-FPOV-WORKAROUND-OVERRIDE: "-cc1"
+// CHECK-NO-FPOV-WORKAROUND-OVERRIDE-NOT: "-ffp-cast-overflow-workaround"
+