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Respect MemoryAccessVolatileMask on OpLoad to prevent forwarding #2

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Respect MemoryAccessVolatileMask on OpLoad to prevent forwarding #2

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846c058
Respect MemoryAccessVolatileMask on OpLoad to prevent forwarding
hughperkins Mar 3, 2026
2dcbe8d
Add regression tests for volatile PhysicalStorageBuffer load
hughperkins Mar 3, 2026
6933ffc
Update spirv_glsl.cpp
hughperkins Mar 23, 2026
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19 changes: 19 additions & 0 deletions reference/shaders-msl-no-opt/asm/comp/volatile-phys-buf-load-no-forward.asm.msl24.comp
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@@ -0,0 +1,19 @@
#include <metal_stdlib>
#include <simd/simd.h>

using namespace metal;

struct Registers
{
ulong addr;
ulong addr2;
};

kernel void main0(constant Registers& registers [[buffer(0)]])
{
device int* _21 = reinterpret_cast<device int*>(registers.addr2);
int _22 = *(reinterpret_cast<device int*>(registers.addr));
*_21 = _22;
*(reinterpret_cast<device int*>(reinterpret_cast<ulong>(_21) + 4ul)) = _22;
}

30 changes: 30 additions & 0 deletions reference/shaders-no-opt/asm/comp/volatile-phys-buf-load-no-forward.nocompat.vk.asm.comp.vk
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#version 450
#if defined(GL_ARB_gpu_shader_int64)
#extension GL_ARB_gpu_shader_int64 : require
#else
#error No extension available for 64-bit integers.
#endif
#extension GL_EXT_buffer_reference2 : require
layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;

layout(buffer_reference) buffer intPointer;

layout(buffer_reference, buffer_reference_align = 4) buffer intPointer
{
int value;
};

layout(push_constant, std430) uniform Registers
{
uint64_t addr;
uint64_t addr2;
} registers;

void main()
{
intPointer _21 = intPointer(registers.addr2);
int _22 = intPointer(registers.addr).value;
_21.value = _22;
intPointer(uint64_t(_21) + 4ul).value = _22;
}

60 changes: 60 additions & 0 deletions shaders-msl-no-opt/asm/comp/volatile-phys-buf-load-no-forward.asm.msl24.comp
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; SPIR-V
; Version: 1.0
; Generator: Khronos Glslang Reference Front End; 10
; Bound: 40
; Schema: 0
OpCapability Shader
OpCapability Int64
OpCapability PhysicalStorageBufferAddresses
OpExtension "SPV_KHR_physical_storage_buffer"
%1 = OpExtInstImport "GLSL.std.450"
OpMemoryModel PhysicalStorageBuffer64 GLSL450
OpEntryPoint GLCompute %main "main"
OpExecutionMode %main LocalSize 1 1 1
OpName %main "main"
OpName %Registers "Registers"
OpMemberName %Registers 0 "addr"
OpMemberName %Registers 1 "addr2"
OpName %registers "registers"
OpMemberDecorate %Registers 0 Offset 0
OpMemberDecorate %Registers 1 Offset 8
OpDecorate %Registers Block
%void = OpTypeVoid
%3 = OpTypeFunction %void
%int = OpTypeInt 32 1
%ulong = OpTypeInt 64 0
%Registers = OpTypeStruct %ulong %ulong
%_ptr_PushConstant_Registers = OpTypePointer PushConstant %Registers
%registers = OpVariable %_ptr_PushConstant_Registers PushConstant
%int_0 = OpConstant %int 0
%int_1 = OpConstant %int 1
%ulong_4 = OpConstant %ulong 4
%_ptr_PushConstant_ulong = OpTypePointer PushConstant %ulong
%_ptr_PhysicalStorageBuffer_int = OpTypePointer PhysicalStorageBuffer %int
%main = OpFunction %void None %3
%5 = OpLabel

%pc0 = OpAccessChain %_ptr_PushConstant_ulong %registers %int_0
%addr0 = OpLoad %ulong %pc0
%src_p = OpConvertUToPtr %_ptr_PhysicalStorageBuffer_int %addr0

%pc1 = OpAccessChain %_ptr_PushConstant_ulong %registers %int_1
%addr1 = OpLoad %ulong %pc1
%dst_p = OpConvertUToPtr %_ptr_PhysicalStorageBuffer_int %addr1

; Volatile load from src — must NOT be forwarded
%ld = OpLoad %int %src_p Volatile|Aligned 4

; Store the loaded value into dst (first use)
OpStore %dst_p %ld Aligned 4

; Compute dst + 1 element via integer arithmetic
%dst_u64 = OpConvertPtrToU %ulong %dst_p
%dst_plus4 = OpIAdd %ulong %dst_u64 %ulong_4
%dst_p2 = OpConvertUToPtr %_ptr_PhysicalStorageBuffer_int %dst_plus4

; Store the same loaded value again (second use)
OpStore %dst_p2 %ld Aligned 4

OpReturn
OpFunctionEnd
60 changes: 60 additions & 0 deletions shaders-no-opt/asm/comp/volatile-phys-buf-load-no-forward.nocompat.vk.asm.comp
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Original file line number Diff line number Diff line change
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; SPIR-V
; Version: 1.0
; Generator: Khronos Glslang Reference Front End; 10
; Bound: 40
; Schema: 0
OpCapability Shader
OpCapability Int64
OpCapability PhysicalStorageBufferAddresses
OpExtension "SPV_KHR_physical_storage_buffer"
%1 = OpExtInstImport "GLSL.std.450"
OpMemoryModel PhysicalStorageBuffer64 GLSL450
OpEntryPoint GLCompute %main "main"
OpExecutionMode %main LocalSize 1 1 1
OpName %main "main"
OpName %Registers "Registers"
OpMemberName %Registers 0 "addr"
OpMemberName %Registers 1 "addr2"
OpName %registers "registers"
OpMemberDecorate %Registers 0 Offset 0
OpMemberDecorate %Registers 1 Offset 8
OpDecorate %Registers Block
%void = OpTypeVoid
%3 = OpTypeFunction %void
%int = OpTypeInt 32 1
%ulong = OpTypeInt 64 0
%Registers = OpTypeStruct %ulong %ulong
%_ptr_PushConstant_Registers = OpTypePointer PushConstant %Registers
%registers = OpVariable %_ptr_PushConstant_Registers PushConstant
%int_0 = OpConstant %int 0
%int_1 = OpConstant %int 1
%ulong_4 = OpConstant %ulong 4
%_ptr_PushConstant_ulong = OpTypePointer PushConstant %ulong
%_ptr_PhysicalStorageBuffer_int = OpTypePointer PhysicalStorageBuffer %int
%main = OpFunction %void None %3
%5 = OpLabel

%pc0 = OpAccessChain %_ptr_PushConstant_ulong %registers %int_0
%addr0 = OpLoad %ulong %pc0
%src_p = OpConvertUToPtr %_ptr_PhysicalStorageBuffer_int %addr0

%pc1 = OpAccessChain %_ptr_PushConstant_ulong %registers %int_1
%addr1 = OpLoad %ulong %pc1
%dst_p = OpConvertUToPtr %_ptr_PhysicalStorageBuffer_int %addr1

; Volatile load from src — must NOT be forwarded
%ld = OpLoad %int %src_p Volatile|Aligned 4

; Store the loaded value into dst (first use)
OpStore %dst_p %ld Aligned 4

; Compute dst + 1 element via integer arithmetic
%dst_u64 = OpConvertPtrToU %ulong %dst_p
%dst_plus4 = OpIAdd %ulong %dst_u64 %ulong_4
%dst_p2 = OpConvertUToPtr %_ptr_PhysicalStorageBuffer_int %dst_plus4

; Store the same loaded value again (second use)
OpStore %dst_p2 %ld Aligned 4

OpReturn
OpFunctionEnd
8 changes: 8 additions & 0 deletions spirv_glsl.cpp
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Expand Up @@ -12429,6 +12429,14 @@ void CompilerGLSL::emit_instruction(const Instruction &instruction)
// If an expression is mutable and forwardable, we speculate that it is immutable.
bool forward = should_forward(ptr) && forced_temporaries.find(id) == end(forced_temporaries);

// Volatile memory access requires the value be read exactly once from
// memory. Do not forward the expression so that re-evaluation at each
// use site cannot re-read potentially modified memory.
// FIXME: To force implementations to actually respect the volatile nature of the load,
// the block itself must be marked volatile, or VulkanMM is used to do an explicit volatile load.
if (forward && length >= 4 && (ops[3] & MemoryAccessVolatileMask) != 0)
forward = false;

// If loading a non-native row-major matrix, mark the expression as need_transpose.
bool need_transpose = false;
bool old_need_transpose = false;
Expand Down