add support for leading_zeros and trailing_zeros #213

Firestar99 · 2025-01-29T15:24:38Z

In main they require some obscure Intel Extension. This PR changes them to use the GLSL.std.450 functions FindILsb and FindUMsb, which do the exact same thing. Though they slightly mismatch when passing in 0, rust expects the bit count of the type but they return !0/-1. This requires a branch, making this a lot more complicated to implement, as I don't know how to split the function down the middle into two different blocks.
They are also limited to 32 bits, which I'm currently ignoring.

Currently panics during linking, probably cause I screwed up branching somewhere:

ConsumerError(DetachedInstruction(Some(Instruction { class: Instruction { opname: "Store", opcode: Store, capabilities: [], extensions: [], operands: [LogicalOperand { kind: IdRef, quantifier: One }, LogicalOperand { kind: IdRef, quantifier: One }, LogicalOperand { kind: MemoryAccess, quantifier: ZeroOrOne }] }, result_type: None, result_id: None, operands: [IdRef(114), IdRef(120)] })))

Also has some unused warnings for capability querying that I don't really want to remove yet, as we may need.

Closes #210

Firestar99 · 2025-01-29T15:37:26Z

I basically want this glsl:

#version 450

layout(location=0) flat in uint inbla;
layout(location=0) out uint outbla;

uint trailing_zeros(uint v) {
    uint ret;
    if (v == 0) {
        ret = 32;
    } else {
        ret = findLSB(v);
    }
    return ret;
}

void main() {
    outbla = trailing_zeros(inbla);
}

Which disassembles to:

    %bla_u1_ = OpFunction %uint None %8
          %v = OpFunctionParameter %_ptr_Function_uint
         %11 = OpLabel
        %ret = OpVariable %_ptr_Function_uint Function
         %12 = OpLoad %uint %v
         %15 = OpIEqual %bool %12 %uint_0
               OpSelectionMerge %17 None
               OpBranchConditional %15 %16 %19
         %16 = OpLabel
               OpStore %ret %uint_0
               OpBranch %17
         %19 = OpLabel
         %20 = OpLoad %uint %v
         %22 = OpExtInst %int %1 FindILsb %20
         %23 = OpBitcast %uint %22
               OpStore %ret %23
               OpBranch %17
         %17 = OpLabel
         %24 = OpLoad %uint %ret
               OpReturnValue %24
               OpFunctionEnd

LegNeato · 2025-01-29T20:12:43Z

Sweet! I actually looked at this too, just didn't put my WIP up. Differences between what I did:

Kept the intel stuff, but made it queried / optional.
Used bitshifts and masking to support rather than the glop (I would imagine the glop is faster?). I basically just looked at stack overflow (https://stackoverflow.com/questions/23856596/how-to-count-leading-zeros-in-a-32-bit-unsigned-integer) and hacker's delight.

I pushed a branch. I think the code is largely correct, but it is only lightly tested:

https://github.com/LegNeato/rust-gpu/tree/clz

add support for leading_zeros and trailing_zeros, panics during linking

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add support for leading_zeros and trailing_zeros #213

add support for leading_zeros and trailing_zeros #213

Firestar99 commented Jan 29, 2025 •

edited

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Firestar99 commented Jan 29, 2025 •

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LegNeato commented Jan 29, 2025 •

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add support for leading_zeros and trailing_zeros #213

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Firestar99 commented Jan 29, 2025 • edited Loading

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Firestar99 commented Jan 29, 2025 •

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LegNeato commented Jan 29, 2025 •

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