Change Conditional<> to Use Vector internally

[gtong-nv]

Problem

When using Conditional<float, true> with the SV_Depth semantic, the SPIR-V emitter was generating invalid SPIR-V code. The issue stemmed from Conditional<T, hasValue> being implemented using T[hasValue] (arrays), which caused:

1. Invalid SPIR-V for FragDepth: Vulkan requires gl_FragDepth to be a scalar float, but the array-based implementation resulted in an array type pointer
2. Type layout crashes: Non-constant array sizes in conditionals caused crashes during varying parameter processing when getIntVal() expected a compile-time constant

Related to #9578

Solution

1. Change Conditional Implementation to Use Vectors

Before:

struct Conditional<T, bool hasValue>
{
    internal T storage[hasValue];
    // ...
}

After:

struct Conditional<T, bool hasValue>
{
    internal vector<T, hasValue ? 1 : 0> storage;
    // ...
}

This change:

• Uses vector<T, 1> when hasValue = true (which gets legalized to scalar T)
• Uses vector<T, 0> when hasValue = false (which gets legalized to void)

2. Implement 0-Length Vector Legalization

Added comprehensive support for 0-length vectors in slang-ir-legalize-vector-types.cpp:

• Detection: Added is0Vector() and related helper functions
• Value operations: Operations that create or access 0-vectors are replaced with appropriate values:
  • IRMakeVector / IRMakeVectorFromScalar → void value
  • IRGetElement / IRSwizzle → poison (undefined behavior)
  • IRGetElementPtr → poison
  • IRSwizzledStore → removed (no-op)
• Type replacement: 0-vector types are replaced with IRVoidType

3. Fix Type Layout for Non-Constant Vector Sizes

Added safety checks in slang-type-layout.cpp to handle vectors with non-constant element counts:

auto elementCountVal = context.tryResolveLinkTimeVal(vecType->getElementCount());
if (!as<ConstantIntVal>(elementCountVal))
{
    // Fall back to default layout, defer to IR legalization
    RefPtr<TypeLayout> typeLayout = new TypeLayout();
    typeLayout->type = type;
    typeLayout->rules = rules;
    return TypeLayoutResult(typeLayout, element.info);
}

This prevents crashes in two locations:

• General type layout creation (line ~5346)
• Varying parameter layout creation (line ~6052)

4. Move Vector Legalization Pass Earlier

Moved legalizeVectorTypes to run immediately after legalizeEmptyArray and before resource/existential type legalization. This ensures:

• 0-vectors are eliminated before they can cause issues in downstream passes
• Consistent with the empty array handling approach

5. Update Validation

Updated slang-ir-validate.cpp to allow 0-length vectors as they are now used internally for conditional varying values and will be legalized away.

Testing

• ✅ Added tests/bugs/conditional-sv-depth.slang - Verifies correct SPIR-V generation for Conditional<float, true> with SV_Depth
• ✅ Updated tests/diagnostics/invalid-vector-element-count.slang - Removed error check for 0-length vectors as they're now valid internally
• ✅ Existing tests pass with the new implementation

Impact

• Breaking Changes: None for user code - this is an internal implementation detail
• Performance: Minimal - 1-vectors and 0-vectors are legalized to more efficient representations
• Targets: All targets benefit from proper legalization, especially SPIR-V/Vulkan

Key Files Changed

• source/slang/core.meta.slang - Conditional implementation change
• source/slang/slang-ir-legalize-vector-types.cpp - 0-vector legalization
• source/slang/slang-type-layout.cpp - Non-constant vector size handling
• source/slang/slang-emit.cpp - Pass ordering
• source/slang/slang-ir-validate.cpp - Validation update

[Copilot]

Pull request overview

This PR fixes a SPIR-V validation error that occurs when using Conditional<float, true> with the SV_Depth semantic. The issue arises because Conditional<T, true> internally expands to a single-element array T[1], but Vulkan's SPIR-V specification requires FragDepth to be a scalar float, not an array.

Changes:

• Added a new legalization pass legalizeDepthOutputTypes() that transforms depth-related outputs from pointer-to-single-element-array to pointer-to-scalar
• Added helper function isDepthSystemValue() to identify depth-related system value semantics
• Added test case to verify the fix generates valid SPIR-V with scalar FragDepth

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/bugs/conditional-sv-depth.slang	Regression test that verifies Conditional<float, true> with SV_Depth generates valid SPIR-V with scalar FragDepth variable
source/slang/slang-ir-spirv-legalize.cpp	Implements legalization pass to unwrap single-element arrays from depth outputs, fixing store/load instructions to maintain correct types

[gtong-nv]

/format

[slangbot]

🌈 Formatted, please merge the changes from this PR

[jkwak-work]

I am wondering the legalization may need to happen in another place.

slang/source/slang/slang-ir-legalize-varying-params.cpp

Line 3022 in 6534d83

legalizeSystemValueParameters(entryPoint);