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[Flang] Add -ffast-real-mod and direct code for MOD on REAL types #160660

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Merged
merged 15 commits into from
Oct 2, 2025
Merged

[Flang] Add -ffast-real-mod and direct code for MOD on REAL types #160660

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8f9868d
Add first rough implementation of -ffast-real-mod
mjklemm Sep 15, 2025
ecae88c
Add command line flag
mjklemm Sep 15, 2025
85b14eb
Pass -ffast-real-mod via MLIR module attribute to code-gen
mjklemm Sep 22, 2025
d32863a
Clean up code
mjklemm Sep 22, 2025
d06a1ad
Add test
mjklemm Sep 22, 2025
313abd0
Improve test and add kind=16 test
mjklemm Sep 22, 2025
2572cc0
Don't use hard-coded register numbers
mjklemm Sep 23, 2025
7cc56df
Honor -ffast-math when present
mjklemm Sep 23, 2025
ed68857
Remove unwanted changes
mjklemm Sep 25, 2025
afc2063
Format code
mjklemm Sep 25, 2025
3b392c8
Follow suit of the test in flang/Lower/Intrinsics/mod.f90
mjklemm Sep 25, 2025
6d5836c
Address reviewer comments
mjklemm Sep 25, 2025
5c8304d
Add Flang driver check for -ffast-real-mod
mjklemm Sep 25, 2025
52c48db
Add -fno-fast-real-mod
mjklemm Sep 26, 2025
d7beb16
Put the MOD optimization under AFN and add -fno-fast-real-mod
mjklemm Sep 30, 2025
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1 change: 1 addition & 0 deletions clang/include/clang/Driver/Options.td
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Expand Up @@ -2750,6 +2750,7 @@ def fno_unsafe_math_optimizations : Flag<["-"], "fno-unsafe-math-optimizations">
Group<f_Group>;
def fassociative_math : Flag<["-"], "fassociative-math">, Visibility<[ClangOption, FlangOption]>, Group<f_Group>;
def fno_associative_math : Flag<["-"], "fno-associative-math">, Visibility<[ClangOption, FlangOption]>, Group<f_Group>;
def ffast_real_mod : Flag<["-"], "ffast-real-mod">, Visibility<[FlangOption, FC1Option]>, Group<f_Group>;
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@vzakhari vzakhari Sep 25, 2025

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Can you please also add -fno-fast-real-mod?

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@mjklemm mjklemm Sep 29, 2025

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I have added it my local copy for now and push it with my next update.

defm reciprocal_math : BoolFOption<"reciprocal-math",
LangOpts<"AllowRecip">, DefaultFalse,
PosFlag<SetTrue, [], [ClangOption, CC1Option, FC1Option, FlangOption],
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3 changes: 3 additions & 0 deletions clang/lib/Driver/ToolChains/Flang.cpp
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Expand Up @@ -766,6 +766,9 @@ static void addFloatingPointOptions(const Driver &D, const ArgList &Args,

if (ReciprocalMath)
CmdArgs.push_back("-freciprocal-math");

if (Args.hasArg(options::OPT_ffast_real_mod))
CmdArgs.push_back("-ffast-real-mod");
}

static void renderRemarksOptions(const ArgList &Args, ArgStringList &CmdArgs,
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3 changes: 2 additions & 1 deletion flang/include/flang/Support/LangOptions.def
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Expand Up @@ -60,7 +60,8 @@ LANGOPT(OpenMPNoThreadState, 1, 0)
LANGOPT(OpenMPNoNestedParallelism, 1, 0)
/// Use SIMD only OpenMP support.
LANGOPT(OpenMPSimd, 1, false)

/// Enable fast MOD operations for REAL
LANGOPT(FastRealMod, 1, false)
LANGOPT(VScaleMin, 32, 0) ///< Minimum vscale range value
LANGOPT(VScaleMax, 32, 0) ///< Maximum vscale range value

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3 changes: 3 additions & 0 deletions flang/lib/Frontend/CompilerInvocation.cpp
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Expand Up @@ -1424,6 +1424,9 @@ static bool parseFloatingPointArgs(CompilerInvocation &invoc,
opts.setFPContractMode(Fortran::common::LangOptions::FPM_Fast);
}

if (args.hasArg(clang::driver::options::OPT_ffast_real_mod))
opts.FastRealMod = true;

return true;
}

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8 changes: 8 additions & 0 deletions flang/lib/Frontend/FrontendActions.cpp
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Expand Up @@ -277,6 +277,14 @@ bool CodeGenAction::beginSourceFileAction() {
ci.getInvocation().getLangOpts().OpenMPVersion);
}

if (ci.getInvocation().getLangOpts().FastRealMod) {
mlir::ModuleOp mod = lb.getModule();
mod.getOperation()->setAttr(
mlir::StringAttr::get(mod.getContext(),
llvm::Twine{"fir.fast_real_mod"}),
mlir::BoolAttr::get(mod.getContext(), true));
}

// Create a parse tree and lower it to FIR
parseAndLowerTree(ci, lb);

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36 changes: 33 additions & 3 deletions flang/lib/Optimizer/Builder/IntrinsicCall.cpp
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Expand Up @@ -7009,8 +7009,31 @@ mlir::Value IntrinsicLibrary::genMergeBits(mlir::Type resultType,
}

// MOD
static mlir::Value genFastMod(fir::FirOpBuilder &builder, mlir::Location loc,
mlir::Value a, mlir::Value p) {
auto fastmathFlags = mlir::arith::FastMathFlags::contract;
auto fastmathAttr =
mlir::arith::FastMathFlagsAttr::get(builder.getContext(), fastmathFlags);
mlir::Value divResult =
mlir::arith::DivFOp::create(builder, loc, a, p, fastmathAttr);
mlir::Type intType = builder.getIntegerType(
a.getType().getIntOrFloatBitWidth(), /*signed=*/true);
mlir::Value intResult = builder.createConvert(loc, intType, divResult);
mlir::Value cnvResult = builder.createConvert(loc, a.getType(), intResult);
mlir::Value mulResult =
mlir::arith::MulFOp::create(builder, loc, cnvResult, p, fastmathAttr);
mlir::Value subResult =
mlir::arith::SubFOp::create(builder, loc, a, mulResult, fastmathAttr);
return subResult;
}

mlir::Value IntrinsicLibrary::genMod(mlir::Type resultType,
llvm::ArrayRef<mlir::Value> args) {
auto mod = builder.getModule();
bool useFastRealMod = false;
if (auto attr = mod->getAttrOfType<mlir::BoolAttr>("fir.fast_real_mod"))
useFastRealMod = attr.getValue();

assert(args.size() == 2);
if (resultType.isUnsignedInteger()) {
mlir::Type signlessType = mlir::IntegerType::get(
Expand All @@ -7022,9 +7045,16 @@ mlir::Value IntrinsicLibrary::genMod(mlir::Type resultType,
if (mlir::isa<mlir::IntegerType>(resultType))
return mlir::arith::RemSIOp::create(builder, loc, args[0], args[1]);

// Use runtime.
return builder.createConvert(
loc, resultType, fir::runtime::genMod(builder, loc, args[0], args[1]));
if (useFastRealMod && resultType.isFloat()) {
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@vzakhari vzakhari Sep 25, 2025

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I wonder if we can enable the fast MOD under afn FastMathFlag. Is there a reason to control it via a separate option?

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@mjklemm mjklemm Sep 29, 2025

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I was doing that to avoid issues with precision loss that could have come in with the other math functions. After I spoke to the engineers, who are working on the applications, it seems they'd be fine with moving this under the afn flag and do the optimization as part of -ffast-math which is enabled for this application.

However, I'm tempted to keep this as a separate flag and simply enable this MOD optimization it when -ffast-math is present. That makes it more controllable w.r.t. to the other math optimizations. What do you think?

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@vzakhari vzakhari Sep 29, 2025

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It sounds okay to me. I would recommend enabling this optimization whenever afn is set (i.e. under -ffast-math or under -fapprox-func), and allow to override this with -fno-fast-real-mod.

// If fast MOD for REAL has been requested, generate less precise,
// but faster code directly.
return builder.createConvert(loc, resultType,
genFastMod(builder, loc, args[0], args[1]));
} else {
// Use runtime.
return builder.createConvert(
loc, resultType, fir::runtime::genMod(builder, loc, args[0], args[1]));
}
}

// MODULO
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77 changes: 77 additions & 0 deletions flang/test/Lower/Intrinsics/fast-real-mod.f90
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@@ -0,0 +1,77 @@
! RUN: %flang_fc1 -ffast-real-mod -emit-mlir -o - %s | FileCheck %s --check-prefixes=CHECK%if target=x86_64{{.*}} %{,CHECK-KIND10%}%if flang-supports-f128-math %{,CHECK-KIND16%}
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@tblah tblah Sep 25, 2025

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I think using kinds which are not enabled for a particular target is a compile error. Please could you put the checks for kind 10 and kind 16 in separate files and skip the whole thing if the types are not supported.

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@mjklemm mjklemm Sep 25, 2025

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I was trying to follow what mod.f90 did. Alas, I failed to do everything that test did. Now the test in this PR does it the same way. Hope that's OK and an acceptable way how I have addressed your comment.

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@tblah tblah Sep 29, 2025

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That test is using lines like

integer, parameter :: kind16 = merge(16, 4, selected_real_kind(p=33).eq.16)
real(kind16) :: r, a, p

This make sure that on systems that don't support real(16) it will instead use real(4).


! CHECK: module attributes {{{.*}}fir.fast_real_mod = true{{.*}}}

! CHECK-LABEL: @_QPmod_real4
subroutine mod_real4(r, a, p)
implicit none
real(kind=4) :: r, a, p
! CHECK: %[[A:.*]] = fir.declare{{.*}}a"
! CHECK: %[[P:.*]] = fir.declare{{.*}}p"
! CHECK: %[[R:.*]] = fir.declare{{.*}}r"
! CHECK: %[[A_LOAD:.*]] = fir.load %[[A]]
! CHECK: %[[P_LOAD:.*]] = fir.load %[[P]]
! CHECK: %[[DIV:.*]] = arith.divf %[[A_LOAD]], %[[P_LOAD]] fastmath<contract> : f32
! CHECK: %[[CV1:.*]] = fir.convert %[[DIV]] : (f32) -> si32
! CHECK: %[[CV2:.*]] = fir.convert %[[CV1]] : (si32) -> f32
! CHECK: %[[MUL:.*]] = arith.mulf %[[CV2]], %[[P_LOAD]] fastmath<contract> : f32
! CHECK: %[[SUB:.*]] = arith.subf %[[A_LOAD]], %[[MUL]] fastmath<contract> : f32
! CHECK: fir.store %[[SUB]] to %[[R]] : !fir.ref<f32>
r = mod(a, p)
end subroutine mod_real4

! CHECK-LABEL: @_QPmod_real8
subroutine mod_real8(r, a, p)
implicit none
real(kind=8) :: r, a, p
! CHECK: %[[A:.*]] = fir.declare{{.*}}a"
! CHECK: %[[P:.*]] = fir.declare{{.*}}p"
! CHECK: %[[R:.*]] = fir.declare{{.*}}r"
! CHECK: %[[A_LOAD:.*]] = fir.load %[[A]]
! CHECK: %[[P_LOAD:.*]] = fir.load %[[P]]
! CHECK: %[[DIV:.*]] = arith.divf %[[A_LOAD]], %[[P_LOAD]] fastmath<contract> : f64
! CHECK: %[[CV1:.*]] = fir.convert %[[DIV]] : (f64) -> si64
! CHECK: %[[CV2:.*]] = fir.convert %[[CV1]] : (si64) -> f64
! CHECK: %[[MUL:.*]] = arith.mulf %[[CV2]], %[[P_LOAD]] fastmath<contract> : f64
! CHECK: %[[SUB:.*]] = arith.subf %[[A_LOAD]], %[[MUL]] fastmath<contract> : f64
! CHECK: fir.store %[[SUB]] to %[[R]] : !fir.ref<f64>
r = mod(a, p)
end subroutine mod_real8

! CHECK-LABEL: @_QPmod_real10
subroutine mod_real10(r, a, p)
implicit none
integer, parameter :: kind10 = merge(10, 4, selected_real_kind(p=18).eq.10)
real(kind=kind10) :: r, a, p
! CHECK-KIND10: %[[A:.*]] = fir.declare{{.*}}a"
! CHECK-KIND10: %[[P:.*]] = fir.declare{{.*}}p"
! CHECK-KIND10: %[[R:.*]] = fir.declare{{.*}}r"
! CHECK-KIND10: %[[A_LOAD:.*]] = fir.load %[[A]]
! CHECK-KIND10: %[[P_LOAD:.*]] = fir.load %[[P]]
! CHECK-KIND10: %[[DIV:.*]] = arith.divf %[[A_LOAD]], %[[P_LOAD]] fastmath<contract> : f80
! CHECK-KIND10: %[[CV1:.*]] = fir.convert %[[DIV]] : (f80) -> si80
! CHECK-KIND10: %[[CV2:.*]] = fir.convert %[[CV1]] : (si80) -> f80
! CHECK-KIND10: %[[MUL:.*]] = arith.mulf %[[CV2]], %[[P_LOAD]] fastmath<contract> : f80
! CHECK-KIND10: %[[SUB:.*]] = arith.subf %[[A_LOAD]], %[[MUL]] fastmath<contract> : f80
! CHECK-KIND10: fir.store %[[SUB]] to %[[R]] : !fir.ref<f80>
r = mod(a, p)
end subroutine mod_real10

! CHECK-LABEL: @_QPmod_real16
subroutine mod_real16(r, a, p)
implicit none
integer, parameter :: kind16 = merge(16, 4, selected_real_kind(p=33).eq.16)
real(kind=kind16) :: r, a, p
! CHECK-KIND16: %[[A:.*]] = fir.declare{{.*}}a"
! CHECK-KIND16: %[[P:.*]] = fir.declare{{.*}}p"
! CHECK-KIND16: %[[R:.*]] = fir.declare{{.*}}r"
! CHECK-KIND16: %[[A_LOAD:.*]] = fir.load %[[A]]
! CHECK-KIND16: %[[P_LOAD:.*]] = fir.load %[[P]]
! CHECK-KIND16: %[[DIV:.*]] = arith.divf %[[A_LOAD]], %[[P_LOAD]] fastmath<contract> : f128
! CHECK-KIND16: %[[CV1:.*]] = fir.convert %[[DIV]] : (f128) -> si128
! CHECK-KIND16: %[[CV2:.*]] = fir.convert %[[CV1]] : (si128) -> f128
! CHECK-KIND16: %[[MUL:.*]] = arith.mulf %[[CV2]], %[[P_LOAD]] fastmath<contract> : f128
! CHECK-KIND16: %[[SUB:.*]] = arith.subf %[[A_LOAD]], %[[MUL]] fastmath<contract> : f128
! CHECK-KIND16: fir.store %[[SUB]] to %[[R]] : !fir.ref<f128>
r = mod(a, p)
end subroutine mod_real16