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[libclc] Refine __clc_fp*_subnormals_supported and __clc_flush_denormal_if_not_supported #157633

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[libclc] Refine __clc_fp*_subnormals_supported and __clc_flush_denormal_if_not_supported #157633

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7e2d210
[libclc] Refine __clc_fp*_subnormals_supported and __clc_flush_denorm…
wenju-he Sep 9, 2025
96ec9dc
Apply suggestions from code review
wenju-he Sep 9, 2025
d52fcdb
use __builtin_elementwise_canonicalize
wenju-he Sep 9, 2025
4608f77
set -fdenormal-fp-math-f32=dynamic build flag globally
wenju-he Sep 9, 2025
3f665ce
rename __clc_flush_denormal_if_not_supportedto __clc_soft_flush_deno…
wenju-he Sep 10, 2025
7b290a2
delete clc_sw_fma
wenju-he Oct 3, 2025
3da9705
-fdenormal-fp-math-f32 -> -fdenormal-fp-math
wenju-he Oct 3, 2025
7d21a1a
remove -Xclang before -fdenormal-fp-math=dynamic
wenju-he Oct 3, 2025
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19 changes: 1 addition & 18 deletions libclc/CMakeLists.txt
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Expand Up @@ -41,8 +41,6 @@ set( LIBCLC_MIN_LLVM 3.9.0 )
set( LIBCLC_TARGETS_TO_BUILD "all"
CACHE STRING "Semicolon-separated list of libclc targets to build, or 'all'." )

option( ENABLE_RUNTIME_SUBNORMAL "Enable runtime linking of subnormal support." OFF )

option(
LIBCLC_USE_SPIRV_BACKEND "Build SPIR-V targets with the SPIR-V backend." OFF
)
Expand Down Expand Up @@ -231,19 +229,6 @@ set( tahiti_aliases pitcairn verde oland hainan bonaire kabini kaveri hawaii
configure_file( libclc.pc.in libclc.pc @ONLY )
install( FILES ${CMAKE_CURRENT_BINARY_DIR}/libclc.pc DESTINATION "${CMAKE_INSTALL_DATADIR}/pkgconfig" )

if( ENABLE_RUNTIME_SUBNORMAL )
foreach( file IN ITEMS subnormal_use_default subnormal_disable )
link_bc(
TARGET ${file}
INPUTS ${CMAKE_CURRENT_SOURCE_DIR}/opencl/lib/generic/${file}.ll
)
install(
FILES $<TARGET_PROPERTY:${file},TARGET_FILE>
DESTINATION "${CMAKE_INSTALL_DATADIR}/clc"
)
endforeach()
endif()

find_package( Python3 REQUIRED COMPONENTS Interpreter )
file( TO_CMAKE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/utils/gen_convert.py script_loc )
add_custom_command(
Expand Down Expand Up @@ -371,9 +356,6 @@ foreach( t ${LIBCLC_TARGETS_TO_BUILD} )
list( APPEND opencl_gen_files clspv-convert.cl )
else()
list( APPEND opencl_gen_files convert.cl )
if ( NOT ENABLE_RUNTIME_SUBNORMAL )
list( APPEND opencl_lib_files opencl/lib/generic/subnormal_use_default.ll )
endif()
endif()
endif()

Expand Down Expand Up @@ -430,6 +412,7 @@ foreach( t ${LIBCLC_TARGETS_TO_BUILD} )
# Error on undefined macros
-Werror=undef
-fdiscard-value-names
-Xclang -fdenormal-fp-math-f32=dynamic
)

if( NOT "${cpu}" STREQUAL "" )
Expand Down
1 change: 0 additions & 1 deletion libclc/clc/include/clc/math/clc_subnormal_config.h
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Expand Up @@ -10,7 +10,6 @@

#include <clc/clcfunc.h>

_CLC_DECL bool __clc_subnormals_disabled();
_CLC_DECL bool __clc_fp16_subnormals_supported();
_CLC_DECL bool __clc_fp32_subnormals_supported();
_CLC_DECL bool __clc_fp64_subnormals_supported();
Expand Down
13 changes: 5 additions & 8 deletions libclc/clc/include/clc/math/math.h
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Expand Up @@ -11,7 +11,6 @@

#include <clc/clc_as_type.h>
#include <clc/clcfunc.h>
#include <clc/math/clc_subnormal_config.h>

#define SNAN 0x001
#define QNAN 0x002
Expand Down Expand Up @@ -66,13 +65,11 @@ bool __attribute__((noinline)) __clc_runtime_has_hw_fma32(void);
#define LOG_MAGIC_NUM_SP32 (1 + NUMEXPBITS_SP32 - EXPBIAS_SP32)

_CLC_OVERLOAD _CLC_INLINE float __clc_flush_denormal_if_not_supported(float x) {
int ix = __clc_as_int(x);
if (!__clc_fp32_subnormals_supported() && ((ix & EXPBITS_SP32) == 0) &&
((ix & MANTBITS_SP32) != 0)) {
ix &= SIGNBIT_SP32;
x = __clc_as_float(ix);
}
return x;
// Avoid calling __clc_fp32_subnormals_supported here: it uses
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You might have less trouble just using canonicalize for now and trying to relax it later

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You might have less trouble just using canonicalize for now and trying to relax it later

do you mean reverting __clc_soft_flush_denormal to use __clc_fp32_subnormals_supported which uses llvm.canonicalize, or just replacing use of __clc_fp32_subnormals_supported with __builtin_elementwise_canonicalize?

// llvm.canonicalize, which quiets sNaN.
return __builtin_fabsf(x) < 0x1p-149f
? __builtin_elementwise_copysign(0.0f, x)
: x;
Comment on lines 59 to 63
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This function no longer matches the name or behavior. This is an unconditional flush to zero

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If denormal is not supported

  • returns signed zero for denormal input since __builtin_fabsf result is 0
  • return x if x is not denormal

If denormal is supported

  • returns x as is regardless if x is denormal or not.

So this function only flushes to zero if denormal is not supported, right?

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There's no support check here. __builtin_fabsf(x) < 0x1p-149f will be true for 0 or a denormal value, regardless of whether the fcmp flushes the input

Also you can use __builtin_elementwise_abs to be consistently using elementwise builtins

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There's no support check here. __builtin_fabsf(x) < 0x1p-149f will be true for 0 or a denormal value, regardless of whether the fcmp flushes the input

Also you can use __builtin_elementwise_abs to be consistently using elementwise builtins

thanks @arsenm. Now I see what you mean. Renamed __clc_flush_denormal_if_not_supported to __clc_soft_flush_denormal in 3f665ce and use __builtin_elementwise_abs

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Renamed __clc_flush_denormal_if_not_supported to __clc_soft_flush_denormal in 3f665ce and use __builtin_elementwise_abs

kindly ping @arsenm for review.

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The name is now right, but the usage is not. You do not want to unconditionally force a denormal flush, pretty much anywhere.

I also believe most of the explicit flushes / canonicalizes in the math function implementations can be deleted with some shuffling around. I removed most of these in the rocm-device-libs implementations a few years ago. A few more remain but I believe they can also be avoided

}

#ifdef cl_khr_fp64
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1 change: 1 addition & 0 deletions libclc/clc/lib/generic/SOURCES
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Expand Up @@ -137,6 +137,7 @@ math/clc_sincos_helpers.cl
math/clc_sinh.cl
math/clc_sinpi.cl
math/clc_sqrt.cl
math/clc_subnormal_config.cl
math/clc_sw_fma.cl
math/clc_tables.cl
math/clc_tan.cl
Expand Down
1 change: 0 additions & 1 deletion libclc/clc/lib/generic/math/clc_exp10.cl
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Expand Up @@ -11,7 +11,6 @@
#include <clc/math/clc_fma.h>
#include <clc/math/clc_ldexp.h>
#include <clc/math/clc_mad.h>
#include <clc/math/clc_subnormal_config.h>
#include <clc/math/math.h>
#include <clc/math/tables.h>
#include <clc/relational/clc_isnan.h>
Expand Down
1 change: 0 additions & 1 deletion libclc/clc/lib/generic/math/clc_hypot.cl
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Expand Up @@ -12,7 +12,6 @@
#include <clc/math/clc_fma.h>
#include <clc/math/clc_mad.h>
#include <clc/math/clc_sqrt.h>
#include <clc/math/clc_subnormal_config.h>
#include <clc/math/math.h>
#include <clc/relational/clc_isnan.h>
#include <clc/shared/clc_clamp.h>
Expand Down
1 change: 0 additions & 1 deletion libclc/clc/lib/generic/math/clc_pow.cl
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Expand Up @@ -12,7 +12,6 @@
#include <clc/math/clc_fma.h>
#include <clc/math/clc_ldexp.h>
#include <clc/math/clc_mad.h>
#include <clc/math/clc_subnormal_config.h>
#include <clc/math/math.h>
#include <clc/math/tables.h>
#include <clc/relational/clc_select.h>
Expand Down
1 change: 0 additions & 1 deletion libclc/clc/lib/generic/math/clc_pown.cl
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Expand Up @@ -12,7 +12,6 @@
#include <clc/math/clc_fma.h>
#include <clc/math/clc_ldexp.h>
#include <clc/math/clc_mad.h>
#include <clc/math/clc_subnormal_config.h>
#include <clc/math/math.h>
#include <clc/math/tables.h>
#include <clc/relational/clc_select.h>
Expand Down
1 change: 0 additions & 1 deletion libclc/clc/lib/generic/math/clc_powr.cl
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Expand Up @@ -12,7 +12,6 @@
#include <clc/math/clc_fma.h>
#include <clc/math/clc_ldexp.h>
#include <clc/math/clc_mad.h>
#include <clc/math/clc_subnormal_config.h>
#include <clc/math/math.h>
#include <clc/math/tables.h>
#include <clc/relational/clc_select.h>
Expand Down
1 change: 0 additions & 1 deletion libclc/clc/lib/generic/math/clc_remquo.cl
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Expand Up @@ -12,7 +12,6 @@
#include <clc/math/clc_floor.h>
#include <clc/math/clc_fma.h>
#include <clc/math/clc_ldexp.h>
#include <clc/math/clc_subnormal_config.h>
#include <clc/math/clc_trunc.h>
#include <clc/math/math.h>
#include <clc/shared/clc_max.h>
Expand Down
46 changes: 46 additions & 0 deletions libclc/clc/lib/generic/math/clc_subnormal_config.cl
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@@ -0,0 +1,46 @@
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include <clc/internal/clc.h>
#include <clc/math/clc_subnormal_config.h>

#ifdef cl_khr_fp16
#pragma OPENCL EXTENSION cl_khr_fp16 : enable
_CLC_DEF bool __clc_fp16_subnormals_supported() {
#ifdef CLC_SPIRV
// SPIR-V doesn't support llvm.canonicalize for now.
return false;
#else
return !__builtin_isfpclass(__builtin_elementwise_canonicalize(0x1p-24h),
__FPCLASS_POSZERO);
#endif
}
#endif // cl_khr_fp16

_CLC_DEF bool __clc_fp32_subnormals_supported() {
#ifdef CLC_SPIRV
// SPIR-V doesn't support llvm.canonicalize for now.
return false;
#else
return !__builtin_isfpclass(__builtin_elementwise_canonicalize(0x1p-149f),
__FPCLASS_POSZERO);
#endif
}

#ifdef cl_khr_fp64
#pragma OPENCL EXTENSION cl_khr_fp64 : enable
_CLC_DEF bool __clc_fp64_subnormals_supported() {
#ifdef CLC_SPIRV
// SPIR-V doesn't support llvm.canonicalize for now.
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Can you just fix that instead of special casing it here? It's not difficult to implement

return false;
#else
return !__builtin_isfpclass(__builtin_elementwise_canonicalize(0x1p-1074),
__FPCLASS_POSZERO);
#endif
}
#endif // cl_khr_fp64
1 change: 0 additions & 1 deletion libclc/opencl/lib/clspv/SOURCES
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@@ -1,6 +1,5 @@
math/fma.cl
shared/vstore_half.cl
subnormal_config.cl
../generic/geometric/distance.cl
../generic/geometric/length.cl
../generic/math/acos.cl
Expand Down
16 changes: 0 additions & 16 deletions libclc/opencl/lib/clspv/subnormal_config.cl
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2 changes: 0 additions & 2 deletions libclc/opencl/lib/generic/SOURCES
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@@ -1,5 +1,3 @@
subnormal_config.cl
subnormal_helper_func.ll
async/async_work_group_copy.cl
async/async_work_group_strided_copy.cl
async/prefetch.cl
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18 changes: 0 additions & 18 deletions libclc/opencl/lib/generic/subnormal_config.cl
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9 changes: 0 additions & 9 deletions libclc/opencl/lib/generic/subnormal_disable.ll
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16 changes: 0 additions & 16 deletions libclc/opencl/lib/generic/subnormal_helper_func.ll
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9 changes: 0 additions & 9 deletions libclc/opencl/lib/generic/subnormal_use_default.ll
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1 change: 0 additions & 1 deletion libclc/opencl/lib/spirv/SOURCES
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@@ -1,4 +1,3 @@
subnormal_config.cl
../generic/async/async_work_group_strided_copy.cl
../generic/async/wait_group_events.cl
../generic/common/degrees.cl
Expand Down
16 changes: 0 additions & 16 deletions libclc/opencl/lib/spirv/subnormal_config.cl
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