[PATCH] D77484: [Vector] Pass VectLib to LTO backend so TLI build correct vector function list

Sun Apr 5 10:08:34 PDT 2020

wenlei added a comment.

In D77484#1962445 <https://reviews.llvm.org/D77484#1962445>, @tejohnson wrote:

> We're trying to move towards encoding all of this in the IR. And in fact, I recently implemented a series of patches to make the TLI to be built per-function, and along with some patches from @gchatelet to encode -fno-builtin* as function attributes, we now handle that part of the TLII with IR. See D67923 <https://reviews.llvm.org/D67923> which is the last patch in the series. We should encode the vectlib as function attributes similarly, and just thread that through to the TLI.

Thanks for quick look, @tejohnson. Yeah, making vectlib a function attribute would be more consistent with other the way other TLI attributes are handled. However, unlike some other attributes, which really need to be per-function, conceptually, vectlib setting won't be different from function to function in a module. Implementation-wise, even though TLI is now built per-function, there's a `BaselineInfoImpl` within each `TargetLibraryInfo`, that's still shared among functions, and `VectorDescs` which is populated based on vectlib setting belongs to the shared baseline. We could also move `VectorDescs` and its population into per-function part of TLI, but I felt that's a bit overkill, though I don't have a strong opinion on this. So I followed how other module level stuff like `lto-sample-profile` is passed to backend..

Alternatively, we could chose to always run `InjectTLIMappings` in pre-LTO pipeline, even if we don't run vectorizer pre-LTO for ThinLTO, so we always have per-function attribute `vector-function-abi-variant` populated and threaded through to LTO time vectorizer. We will have to make sure the vector function list in TLI (based on vectlib setting) has no other use case though.

(More on TLI, while this change (or a variant of this) enables vectorization that uses math library for ThinLTO w/ legacy PM, there's separate issue with new PM as `InjectTLIMappings` is not scheduled before Vectorization for LTO pipeline. I will send a separate patch for that once we settle on a good way to handle this one.)

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