Update examples in waveballot.md and waveactivecountbits.md #2077
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This example was incorrectly copied from the [spec](https://github.com/microsoft/DirectXShaderCompiler/wiki/Wave-Intrinsics#uint-waveactivecountbits-bool-bbit-), replacing `countbits` with `WaveActiveCountBits`.
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@damyanp : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change. |
tex3d
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Jul 11, 2025
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| ``` syntax | ||
| result = WaveActiveCountBits( WaveActiveBallot( bBit ) ); | ||
| result = countbits( WaveActiveBallot( bBit ) ); |
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This is an improvement, but still not quite correct HLSL. It needs to be more like this:
Suggested change
| result = countbits( WaveActiveBallot( bBit ) ); | |
| uint4 counts = countbits( WaveActiveBallot( bBit ) ); | |
| result = counts.x + counts.y + counts.z + counts.w; |
This is because WaveActiveBallow returns a uint4 in order to have one bit for every potential lane in the widest possible lane width of 128.
tex3d
reviewed
Jul 11, 2025
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| ## Examples | ||
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| This can be implemented more efficiently than a full WaveActiveSum, as described in the following example: |
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Maybe this line should be clarified that this is meant to demonstrate what this operation does:
Suggested change
| This can be implemented more efficiently than a full WaveActiveSum, as described in the following example: | |
| This can be implemented more efficiently than a full WaveActiveSum, in a way similar to this equivalent code: |
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Tried another way to put this, hopefully this is clearer?
damyanp
changed the title
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One example was incorrectly copied from the spec, replacing
countbitswithWaveActiveCountBits. In both cases they were relying on an implicit vector truncation which would result in incorrect results on devices with wave size > 32.