Index: docs/UsersManual.rst =================================================================== --- docs/UsersManual.rst +++ docs/UsersManual.rst @@ -1255,6 +1255,15 @@ 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 or disable a code generation optimization that may convert a + cast of a floating-point value to integer and back to floating-point + into the equivalent of the math libary's 'trunc()' function. This + optimization is disabled by default to avoid problems associated + with relying on the undefined behavior of an overflowing cast, but + that limitation should be considered temporary. + .. option:: -fwhole-program-vtables Enable whole-program vtable optimizations, such as single-implementation Index: include/clang/Driver/Options.td =================================================================== --- include/clang/Driver/Options.td +++ include/clang/Driver/Options.td @@ -1029,6 +1029,11 @@ 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, Flags<[CC1Option]>; +def fno_fp_cast_overflow_workaround : Flag<["-"], + "fno-fp-cast-overflow-workaround">, Group, Flags<[CC1Option]>; + def ffor_scope : Flag<["-"], "ffor-scope">, Group; def fno_for_scope : Flag<["-"], "fno-for-scope">, Group; Index: include/clang/Frontend/CodeGenOptions.def =================================================================== --- include/clang/Frontend/CodeGenOptions.def +++ include/clang/Frontend/CodeGenOptions.def @@ -136,6 +136,11 @@ 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 to preserve behavior for +/// source code that depends on existing -- but undefined -- behavior. +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. Index: lib/CodeGen/CGCall.cpp =================================================================== --- lib/CodeGen/CGCall.cpp +++ lib/CodeGen/CGCall.cpp @@ -1727,6 +1727,9 @@ 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", Index: lib/Driver/ToolChains/Clang.cpp =================================================================== --- lib/Driver/ToolChains/Clang.cpp +++ lib/Driver/ToolChains/Clang.cpp @@ -2241,6 +2241,14 @@ CmdArgs.push_back("-mfpmath"); CmdArgs.push_back(A->getValue()); } + + // Disable a codegen optimization for floating-point casts. This is only + // intended to be temporarily defaulted to 'true' to give users time to fix + // their code. + if (Args.hasFlag(options::OPT_ffp_cast_overflow_workaround, + options::OPT_fno_fp_cast_overflow_workaround, + true)) + CmdArgs.push_back("-ffp-cast-overflow-workaround"); } static void RenderAnalyzerOptions(const ArgList &Args, ArgStringList &CmdArgs, Index: lib/Frontend/CompilerInvocation.cpp =================================================================== --- lib/Frontend/CompilerInvocation.cpp +++ lib/Frontend/CompilerInvocation.cpp @@ -699,6 +699,8 @@ 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); Index: test/CodeGen/no-junk-ftrunc.c =================================================================== --- test/CodeGen/no-junk-ftrunc.c +++ test/CodeGen/no-junk-ftrunc.c @@ -0,0 +1,8 @@ +// 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"{{.*}} + +int main() { + return 0; +} Index: test/Driver/fast-math.c =================================================================== --- test/Driver/fast-math.c +++ 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 passed by default (FIXME). + +// 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: "-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" +