From 91d2ee608d3186b4d0003a8c5eaf9a4debb1a26b Mon Sep 17 00:00:00 2001
From: dan sinclair <[email protected]>
Date: Wed, 19 Aug 2026 09:08:44 -0700
Subject: [PATCH] Check SubgroupBroadcast ID in SPIR-V

When using a `subgroupBroadcast` if the provided `id` is out of range we
return an indeterminant value instead of the broadcast returned value.

Fixed: 546639650
Change-Id: I1678cf899977a377eb9fcd2c71e5a0a1b79dd8f7
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/333795
Commit-Queue: dan sinclair <[email protected]>
Reviewed-by: James Price <[email protected]>
---
diff --git a/src/tint/lang/spirv/writer/raise/builtin_polyfill.cc b/src/tint/lang/spirv/writer/raise/builtin_polyfill.cc
index 07524a8..c847a90 100644
--- a/src/tint/lang/spirv/writer/raise/builtin_polyfill.cc
+++ b/src/tint/lang/spirv/writer/raise/builtin_polyfill.cc
@@ -1320,6 +1320,15 @@
         ir.properties.Add(core::ir::Property::kAllowAnyInputAttachmentIndexType);
     }
 
+    void CreateSubgroupLastIdIfNeeded() {
+        if (subgroup_last_id_) {
+            return;
+        }
+        b.Append(ir.root_block, [&] {
+            subgroup_last_id_ = b.Var<core::AddressSpace::kPrivate, u32>("tint_subgroup_last_id");
+        });
+    }
+
     /// Handles SubgroupShuffle() builtin.
     /// @param builtin the builtin call instruction
     void SubgroupShuffle(core::ir::CoreBuiltinCall* builtin) {
@@ -1336,12 +1345,8 @@
 
         /// Polyfill a `subgroupShuffleX` builtin call with one that has clamped the arg2 param
         auto* shuffle_id = builtin->Args()[1];
-        if (!subgroup_last_id_) {
-            b.Append(ir.root_block, [&] {
-                subgroup_last_id_ =
-                    b.Var<core::AddressSpace::kPrivate, u32>("tint_subgroup_last_id");
-            });
-        }
+        CreateSubgroupLastIdIfNeeded();
+
         b.InsertBefore(builtin, [&] {
             auto* last_id = b.Load(subgroup_last_id_);
             auto* clamp_via_min = b.Min(shuffle_id, last_id);
@@ -1389,8 +1394,26 @@
 
         // For const signed int IDs, compile-time convert to u32 to maintain constness.
         if (id->Type()->IsSignedIntegerScalar()) {
-            builtin->SetArg(1, b.Constant(id->As<core::ir::Constant>()->Value()->ValueAs<u32>()));
+            id = b.Constant(id->As<core::ir::Constant>()->Value()->ValueAs<u32>());
         }
+        builtin->SetArg(1, id);
+
+        CreateSubgroupLastIdIfNeeded();
+
+        core::ir::CoreBuiltinCall* call = nullptr;
+        b.InsertAfter(builtin, [&] {
+            auto* ret_ty = builtin->Result()->Type();
+            auto* result = b.InstructionResult(ret_ty);
+
+            // Replace the uses before we create the new usage.
+            builtin->Result()->ReplaceAllUsesWith(result);
+
+            auto* cmp = b.LessThanEqual(id, b.Load(subgroup_last_id_));
+            call = b.CallWithResult(result, core::BuiltinFn::kSelect, b.Zero(ret_ty), builtin, cmp);
+        });
+
+        // Make sure the select gets converted to proper SPIR-V select
+        Select(call);
     }
 
     /// Handle a QuadBroadcast() builtin.
diff --git a/test/tint/builtins/gen/literal/subgroupBroadcast/02f329.wgsl.expected.spvasm b/test/tint/builtins/gen/literal/subgroupBroadcast/02f329.wgsl.expected.spvasm
index 2d1a399..5acedfd 100644
--- a/test/tint/builtins/gen/literal/subgroupBroadcast/02f329.wgsl.expected.spvasm
+++ b/test/tint/builtins/gen/literal/subgroupBroadcast/02f329.wgsl.expected.spvasm
@@ -4,17 +4,19 @@
 ; SPIR-V
 ; Version: 1.3
 ; Generator: Google Tint Compiler; 1
-; Bound: 26
+; Bound: 38
 ; Schema: 0
                OpCapability Shader
                OpCapability Float16
                OpCapability StorageBuffer16BitAccess
                OpCapability GroupNonUniformBallot
+               OpCapability GroupNonUniformArithmetic
                OpMemoryModel Logical GLSL450
                OpEntryPoint Fragment %fragment_main "fragment_main"
                OpExecutionMode %fragment_main OriginUpperLeft
                OpMemberName %prevent_dce_block 0 "inner"
                OpName %prevent_dce_block "prevent_dce_block"
+               OpName %tint_subgroup_last_id "tint_subgroup_last_id"
                OpName %subgroupBroadcast_02f329 "subgroupBroadcast_02f329"
                OpName %res "res"
                OpName %fragment_main "fragment_main"
@@ -28,30 +30,43 @@
 %prevent_dce_block = OpTypeStruct %v4half
 %_ptr_StorageBuffer_prevent_dce_block = OpTypePointer StorageBuffer %prevent_dce_block
           %1 = OpVariable %_ptr_StorageBuffer_prevent_dce_block StorageBuffer
-          %7 = OpTypeFunction %v4half
        %uint = OpTypeInt 32 0
+%_ptr_Private_uint = OpTypePointer Private %uint
+          %9 = OpConstantNull %uint
+%tint_subgroup_last_id = OpVariable %_ptr_Private_uint Private %9
+         %11 = OpTypeFunction %v4half
      %uint_3 = OpConstant %uint 3
 %half_0x1p_0 = OpConstant %half 0x1p+0
-         %12 = OpConstantComposite %v4half %half_0x1p_0 %half_0x1p_0 %half_0x1p_0 %half_0x1p_0
+         %15 = OpConstantComposite %v4half %half_0x1p_0 %half_0x1p_0 %half_0x1p_0 %half_0x1p_0
      %uint_1 = OpConstant %uint 1
+       %bool = OpTypeBool
+     %v4bool = OpTypeVector %bool 4
+         %24 = OpConstantNull %v4half
 %_ptr_Function_v4half = OpTypePointer Function %v4half
        %void = OpTypeVoid
-         %20 = OpTypeFunction %void
+         %30 = OpTypeFunction %void
 %_ptr_StorageBuffer_v4half = OpTypePointer StorageBuffer %v4half
      %uint_0 = OpConstant %uint 0
-%subgroupBroadcast_02f329 = OpFunction %v4half None %7
-          %8 = OpLabel
+%subgroupBroadcast_02f329 = OpFunction %v4half None %11
+         %12 = OpLabel
         %res = OpVariable %_ptr_Function_v4half Function
-          %9 = OpGroupNonUniformBroadcast %v4half %uint_3 %12 %uint_1
-               OpStore %res %9
-         %17 = OpLoad %v4half %res None
-               OpReturnValue %17
+         %13 = OpGroupNonUniformBroadcast %v4half %uint_3 %15 %uint_1
+         %18 = OpLoad %uint %tint_subgroup_last_id None
+         %19 = OpULessThanEqual %bool %uint_1 %18
+         %22 = OpCompositeConstruct %v4bool %19 %19 %19 %19
+         %23 = OpSelect %v4half %22 %13 %24
+               OpStore %res %23
+         %27 = OpLoad %v4half %res None
+               OpReturnValue %27
                OpFunctionEnd
-%fragment_main = OpFunction %void None %20
-         %21 = OpLabel
-         %22 = OpFunctionCall %v4half %subgroupBroadcast_02f329
-         %23 = OpAccessChain %_ptr_StorageBuffer_v4half %1 %uint_0
-               OpStore %23 %22 None
+%fragment_main = OpFunction %void None %30
+         %31 = OpLabel
+         %32 = OpGroupNonUniformIAdd %uint %uint_3 Reduce %uint_1
+         %33 = OpISub %uint %32 %uint_1
+               OpStore %tint_subgroup_last_id %33 None
+         %34 = OpFunctionCall %v4half %subgroupBroadcast_02f329
+         %35 = OpAccessChain %_ptr_StorageBuffer_v4half %1 %uint_0
+               OpStore %35 %34 None
                OpReturn
                OpFunctionEnd
 ;
@@ -60,17 +75,19 @@
 ; SPIR-V
 ; Version: 1.3
 ; Generator: Google Tint Compiler; 1
-; Bound: 26
+; Bound: 38
 ; Schema: 0
                OpCapability Shader
                OpCapability Float16
                OpCapability StorageBuffer16BitAccess
                OpCapability GroupNonUniformBallot
+               OpCapability GroupNonUniformArithmetic
                OpMemoryModel Logical GLSL450
                OpEntryPoint GLCompute %compute_main "compute_main"
                OpExecutionMode %compute_main LocalSize 1 1 1
                OpMemberName %prevent_dce_block 0 "inner"
                OpName %prevent_dce_block "prevent_dce_block"
+               OpName %tint_subgroup_last_id "tint_subgroup_last_id"
                OpName %subgroupBroadcast_02f329 "subgroupBroadcast_02f329"
                OpName %res "res"
                OpName %compute_main "compute_main"
@@ -84,29 +101,42 @@
 %prevent_dce_block = OpTypeStruct %v4half
 %_ptr_StorageBuffer_prevent_dce_block = OpTypePointer StorageBuffer %prevent_dce_block
           %1 = OpVariable %_ptr_StorageBuffer_prevent_dce_block StorageBuffer
-          %7 = OpTypeFunction %v4half
        %uint = OpTypeInt 32 0
+%_ptr_Private_uint = OpTypePointer Private %uint
+          %9 = OpConstantNull %uint
+%tint_subgroup_last_id = OpVariable %_ptr_Private_uint Private %9
+         %11 = OpTypeFunction %v4half
      %uint_3 = OpConstant %uint 3
 %half_0x1p_0 = OpConstant %half 0x1p+0
-         %12 = OpConstantComposite %v4half %half_0x1p_0 %half_0x1p_0 %half_0x1p_0 %half_0x1p_0
+         %15 = OpConstantComposite %v4half %half_0x1p_0 %half_0x1p_0 %half_0x1p_0 %half_0x1p_0
      %uint_1 = OpConstant %uint 1
+       %bool = OpTypeBool
+     %v4bool = OpTypeVector %bool 4
+         %24 = OpConstantNull %v4half
 %_ptr_Function_v4half = OpTypePointer Function %v4half
        %void = OpTypeVoid
-         %20 = OpTypeFunction %void
+         %30 = OpTypeFunction %void
 %_ptr_StorageBuffer_v4half = OpTypePointer StorageBuffer %v4half
      %uint_0 = OpConstant %uint 0
-%subgroupBroadcast_02f329 = OpFunction %v4half None %7
-          %8 = OpLabel
+%subgroupBroadcast_02f329 = OpFunction %v4half None %11
+         %12 = OpLabel
         %res = OpVariable %_ptr_Function_v4half Function
-          %9 = OpGroupNonUniformBroadcast %v4half %uint_3 %12 %uint_1
-               OpStore %res %9
-         %17 = OpLoad %v4half %res None
-               OpReturnValue %17
+         %13 = OpGroupNonUniformBroadcast %v4half %uint_3 %15 %uint_1
+         %18 = OpLoad %uint %tint_subgroup_last_id None
+         %19 = OpULessThanEqual %bool %uint_1 %18
+         %22 = OpCompositeConstruct %v4bool %19 %19 %19 %19
+         %23 = OpSelect %v4half %22 %13 %24
+               OpStore %res %23
+         %27 = OpLoad %v4half %res None
+               OpReturnValue %27
                OpFunctionEnd
-%compute_main = OpFunction %void None %20
-         %21 = OpLabel
-         %22 = OpFunctionCall %v4half %subgroupBroadcast_02f329
-         %23 = OpAccessChain %_ptr_StorageBuffer_v4half %1 %uint_0
-               OpStore %23 %22 None
+%compute_main = OpFunction %void None %30
+         %31 = OpLabel
+         %32 = OpGroupNonUniformIAdd %uint %uint_3 Reduce %uint_1
+         %33 = OpISub %uint %32 %uint_1
+               OpStore %tint_subgroup_last_id %33 None
+         %34 = OpFunctionCall %v4half %subgroupBroadcast_02f329
+         %35 = OpAccessChain %_ptr_StorageBuffer_v4half %1 %uint_0
+               OpStore %35 %34 None
                OpReturn
                OpFunctionEnd
diff --git a/test/tint/builtins/gen/literal/subgroupBroadcast/07e2d8.wgsl.expected.spvasm b/test/tint/builtins/gen/literal/subgroupBroadcast/07e2d8.wgsl.expected.spvasm
index ca1b932..928f57f 100644
--- a/test/tint/builtins/gen/literal/subgroupBroadcast/07e2d8.wgsl.expected.spvasm
+++ b/test/tint/builtins/gen/literal/subgroupBroadcast/07e2d8.wgsl.expected.spvasm
@@ -4,17 +4,19 @@
 ; SPIR-V
 ; Version: 1.3
 ; Generator: Google Tint Compiler; 1
-; Bound: 24
+; Bound: 34
 ; Schema: 0
                OpCapability Shader
                OpCapability Float16
                OpCapability StorageBuffer16BitAccess
                OpCapability GroupNonUniformBallot
+               OpCapability GroupNonUniformArithmetic
                OpMemoryModel Logical GLSL450
                OpEntryPoint Fragment %fragment_main "fragment_main"
                OpExecutionMode %fragment_main OriginUpperLeft
                OpMemberName %prevent_dce_block 0 "inner"
                OpName %prevent_dce_block "prevent_dce_block"
+               OpName %tint_subgroup_last_id "tint_subgroup_last_id"
                OpName %subgroupBroadcast_07e2d8 "subgroupBroadcast_07e2d8"
                OpName %res "res"
                OpName %fragment_main "fragment_main"
@@ -27,29 +29,40 @@
 %prevent_dce_block = OpTypeStruct %half
 %_ptr_StorageBuffer_prevent_dce_block = OpTypePointer StorageBuffer %prevent_dce_block
           %1 = OpVariable %_ptr_StorageBuffer_prevent_dce_block StorageBuffer
-          %6 = OpTypeFunction %half
        %uint = OpTypeInt 32 0
+%_ptr_Private_uint = OpTypePointer Private %uint
+          %8 = OpConstantNull %uint
+%tint_subgroup_last_id = OpVariable %_ptr_Private_uint Private %8
+         %10 = OpTypeFunction %half
      %uint_3 = OpConstant %uint 3
 %half_0x1p_0 = OpConstant %half 0x1p+0
      %uint_1 = OpConstant %uint 1
+       %bool = OpTypeBool
+%half_0x0p_0 = OpConstant %half 0x0p+0
 %_ptr_Function_half = OpTypePointer Function %half
        %void = OpTypeVoid
-         %18 = OpTypeFunction %void
+         %26 = OpTypeFunction %void
 %_ptr_StorageBuffer_half = OpTypePointer StorageBuffer %half
      %uint_0 = OpConstant %uint 0
-%subgroupBroadcast_07e2d8 = OpFunction %half None %6
-          %7 = OpLabel
+%subgroupBroadcast_07e2d8 = OpFunction %half None %10
+         %11 = OpLabel
         %res = OpVariable %_ptr_Function_half Function
-          %8 = OpGroupNonUniformBroadcast %half %uint_3 %half_0x1p_0 %uint_1
-               OpStore %res %8
-         %15 = OpLoad %half %res None
-               OpReturnValue %15
+         %12 = OpGroupNonUniformBroadcast %half %uint_3 %half_0x1p_0 %uint_1
+         %16 = OpLoad %uint %tint_subgroup_last_id None
+         %17 = OpULessThanEqual %bool %uint_1 %16
+         %19 = OpSelect %half %17 %12 %half_0x0p_0
+               OpStore %res %19
+         %23 = OpLoad %half %res None
+               OpReturnValue %23
                OpFunctionEnd
-%fragment_main = OpFunction %void None %18
-         %19 = OpLabel
-         %20 = OpFunctionCall %half %subgroupBroadcast_07e2d8
-         %21 = OpAccessChain %_ptr_StorageBuffer_half %1 %uint_0
-               OpStore %21 %20 None
+%fragment_main = OpFunction %void None %26
+         %27 = OpLabel
+         %28 = OpGroupNonUniformIAdd %uint %uint_3 Reduce %uint_1
+         %29 = OpISub %uint %28 %uint_1
+               OpStore %tint_subgroup_last_id %29 None
+         %30 = OpFunctionCall %half %subgroupBroadcast_07e2d8
+         %31 = OpAccessChain %_ptr_StorageBuffer_half %1 %uint_0
+               OpStore %31 %30 None
                OpReturn
                OpFunctionEnd
 ;
@@ -58,17 +71,19 @@
 ; SPIR-V
 ; Version: 1.3
 ; Generator: Google Tint Compiler; 1
-; Bound: 24
+; Bound: 34
 ; Schema: 0
                OpCapability Shader
                OpCapability Float16
                OpCapability StorageBuffer16BitAccess
                OpCapability GroupNonUniformBallot
+               OpCapability GroupNonUniformArithmetic
                OpMemoryModel Logical GLSL450
                OpEntryPoint GLCompute %compute_main "compute_main"
                OpExecutionMode %compute_main LocalSize 1 1 1
                OpMemberName %prevent_dce_block 0 "inner"
                OpName %prevent_dce_block "prevent_dce_block"
+               OpName %tint_subgroup_last_id "tint_subgroup_last_id"
                OpName %subgroupBroadcast_07e2d8 "subgroupBroadcast_07e2d8"
                OpName %res "res"
                OpName %compute_main "compute_main"
@@ -81,28 +96,39 @@
 %prevent_dce_block = OpTypeStruct %half
 %_ptr_StorageBuffer_prevent_dce_block = OpTypePointer StorageBuffer %prevent_dce_block
           %1 = OpVariable %_ptr_StorageBuffer_prevent_dce_block StorageBuffer
-          %6 = OpTypeFunction %half
        %uint = OpTypeInt 32 0
+%_ptr_Private_uint = OpTypePointer Private %uint
+          %8 = OpConstantNull %uint
+%tint_subgroup_last_id = OpVariable %_ptr_Private_uint Private %8
+         %10 = OpTypeFunction %half
      %uint_3 = OpConstant %uint 3
 %half_0x1p_0 = OpConstant %half 0x1p+0
      %uint_1 = OpConstant %uint 1
+       %bool = OpTypeBool
+%half_0x0p_0 = OpConstant %half 0x0p+0
 %_ptr_Function_half = OpTypePointer Function %half
        %void = OpTypeVoid
-         %18 = OpTypeFunction %void
+         %26 = OpTypeFunction %void
 %_ptr_StorageBuffer_half = OpTypePointer StorageBuffer %half
      %uint_0 = OpConstant %uint 0
-%subgroupBroadcast_07e2d8 = OpFunction %half None %6
-          %7 = OpLabel
+%subgroupBroadcast_07e2d8 = OpFunction %half None %10
+         %11 = OpLabel
         %res = OpVariable %_ptr_Function_half Function
-          %8 = OpGroupNonUniformBroadcast %half %uint_3 %half_0x1p_0 %uint_1
-               OpStore %res %8
-         %15 = OpLoad %half %res None
-               OpReturnValue %15
+         %12 = OpGroupNonUniformBroadcast %half %uint_3 %half_0x1p_0 %uint_1
+         %16 = OpLoad %uint %tint_subgroup_last_id None
+         %17 = OpULessThanEqual %bool %uint_1 %16
+         %19 = OpSelect %half %17 %12 %half_0x0p_0
+               OpStore %res %19
+         %23 = OpLoad %half %res None
+               OpReturnValue %23
                OpFunctionEnd
-%compute_main = OpFunction %void None %18
-         %19 = OpLabel
-         %20 = OpFunctionCall %half %subgroupBroadcast_07e2d8
-         %21 = OpAccessChain %_ptr_StorageBuffer_half %1 %uint_0
-               OpStore %21 %20 None
+%compute_main = OpFunction %void None %26
+         %27 = OpLabel
+         %28 = OpGroupNonUniformIAdd %uint %uint_3 Reduce %uint_1
+         %29 = OpISub %uint %28 %uint_1
+               OpStore %tint_subgroup_last_id %29 None
+         %30 = OpFunctionCall %half %subgroupBroadcast_07e2d8
+         %31 = OpAccessChain %_ptr_StorageBuffer_half %1 %uint_0
+               OpStore %31 %30 None
                OpReturn
                OpFunctionEnd
diff --git a/test/tint/builtins/gen/literal/subgroupBroadcast/08beca.wgsl.expected.spvasm b/test/tint/builtins/gen/literal/subgroupBroadcast/08beca.wgsl.expected.spvasm
index e8ea2ad..0fd2849 100644
--- a/test/tint/builtins/gen/literal/subgroupBroadcast/08beca.wgsl.expected.spvasm
+++ b/test/tint/builtins/gen/literal/subgroupBroadcast/08beca.wgsl.expected.spvasm
@@ -4,15 +4,17 @@
 ; SPIR-V
 ; Version: 1.3
 ; Generator: Google Tint Compiler; 1
-; Bound: 24
... (file diff truncated)

diff --git a/src/tint/lang/spirv/writer/raise/builtin_polyfill_test.cc b/src/tint/lang/spirv/writer/raise/builtin_polyfill_test.cc
index 59403b2..f889ff2 100644
--- a/src/tint/lang/spirv/writer/raise/builtin_polyfill_test.cc
+++ b/src/tint/lang/spirv/writer/raise/builtin_polyfill_test.cc
@@ -4346,10 +4346,20 @@
     EXPECT_EQ(src, str());
 
     auto* expect = R"(
+$B1: {  # root
+  %tint_subgroup_last_id:ptr<private, u32, read_write> = var undef
+}
+
 %foo = @fragment func():void {
-  $B1: {
-    %2:i32 = subgroupBroadcast 1i, 1u
-    %a:i32 = let %2
+  $B2: {
+    %3:u32 = subgroupAdd 1u
+    %4:u32 = sub %3, 1u
+    store %tint_subgroup_last_id, %4
+    %5:i32 = subgroupBroadcast 1i, 1u
+    %6:u32 = load %tint_subgroup_last_id
+    %7:bool = lte 1u, %6
+    %8:i32 = spirv.select %7, %5, 0i
+    %a:i32 = let %8
     ret
   }
 }