rustc: Allow targets to specify SIMD args are by-val #55024
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The upcoming SIMD support in the wasm target is unique from the other platforms where it's either unconditionally available or not available, there's no halfway where a subsection of the program can use it but no other parts of the program can use it. In this world it's valid for wasm SIMD args to always be passed by value and there's no need to pass them by reference. This commit adds a new custom target specification option `simd_types_indirect` which defaults to `true`, but the wasm backend disables this and sets it to `false`.
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Could you explain this a bit further (I'm not very familiar with the details of wasm)? What exactly determines whether it's available? Is this something inherent to wasm or just the LLVM wasm backend? |
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@bors try for perf |
rustc: Allow targets to specify SIMD args are by-val The upcoming SIMD support in the wasm target is unique from the other platforms where it's either unconditionally available or not available, there's no halfway where a subsection of the program can use it but no other parts of the program can use it. In this world it's valid for wasm SIMD args to always be passed by value and there's no need to pass them by reference. This commit adds a new custom target specification option `simd_types_indirect` which defaults to `true`, but the wasm backend disables this and sets it to `false`.
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@parched on x86/x86_64 we want the ability to compile some parts of the program that optionally use SIMD features that the CPU the executable is running on may not actually have. For example we want the ability to compile AVX2 functionality into a program, but if the CPU doesn't have the functionality then the program can still operate correctly. That has weird implications on ABI which basically forces us to always pass vector types by reference (what we already do today). For wasm, however, such a program is not possible. While the simd128 feature isn't stable in wasm your wasm engine either supports simd128 or not. It's impossible to have a wasm module where only some of it uses simd128 and the rest of it doesn't, and then have it run in an engine that doesn't have simd128 and the program just happens to not execute those pieces. Because the same form of functionality isn't required for wasm (using a program with features on a platform that doesn't support those features), the ABI restrictions also aren't needed on wasm. @Mark-Simulacrum this shouldn't affect any existing code, so I'm not sure what the perf run is for? |
Ok I think I understand. So are you saying that the wasm engine checks/compiles all the code regardless of whether it will actually be run. I.e., you could never have some runtime detection of whether the engine supports simd before executing some simd code? |
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@parched precisely! |
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Oh, I confused this with the other related PR, I believe. |
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☀️ Test successful - status-travis |
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@bors r+ |
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📌 Commit 9562b69 has been approved by |
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rustc: Allow targets to specify SIMD args are by-val The upcoming SIMD support in the wasm target is unique from the other platforms where it's either unconditionally available or not available, there's no halfway where a subsection of the program can use it but no other parts of the program can use it. In this world it's valid for wasm SIMD args to always be passed by value and there's no need to pass them by reference. This commit adds a new custom target specification option `simd_types_indirect` which defaults to `true`, but the wasm backend disables this and sets it to `false`.
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☀️ Test successful - status-appveyor, status-travis |
The upcoming SIMD support in the wasm target is unique from the other
platforms where it's either unconditionally available or not available,
there's no halfway where a subsection of the program can use it but no
other parts of the program can use it. In this world it's valid for wasm
SIMD args to always be passed by value and there's no need to pass them
by reference.
This commit adds a new custom target specification option
simd_types_indirectwhich defaults totrue, but the wasm backenddisables this and sets it to
false.