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Fix argument ABI for overaligned structs on ppc64le #122781

Merged
merged 2 commits into from
Apr 8, 2024
Merged

Fix argument ABI for overaligned structs on ppc64le #122781

merged 2 commits into from
Apr 8, 2024

Commits on Apr 8, 2024

  1. Fix argument ABI for overaligned structs on ppc64le 

    When passing a 16 (or higher) aligned struct by value on ppc64le,
it needs to be passed as an array of `i128` rather than an array
of `i64`. This will force the use of an even starting register.

For the case of a 16 byte struct with alignment 16 it is important
that `[1 x i128]` is used instead of `i128` -- apparently, the
latter will get treated similarly to `[2 x i64]`, not exhibiting
the correct ABI. Add a `force_array` flag to `Uniform` to support
this.

The relevant clang code can be found here:
https://github.com/llvm/llvm-project/blob/fe2119a7b08b6e468b2a67768904ea85b1bf0a45/clang/lib/CodeGen/Targets/PPC.cpp#L878-L884
https://github.com/llvm/llvm-project/blob/fe2119a7b08b6e468b2a67768904ea85b1bf0a45/clang/lib/CodeGen/Targets/PPC.cpp#L780-L784

I think the corresponding psABI wording is this:

> Fixed size aggregates and unions passed by value are mapped to as
> many doublewords of the parameter save area as the value uses in
> memory. Aggregrates and unions are aligned according to their
> alignment requirements. This may result in doublewords being
> skipped for alignment.

In particular the last sentence.

Fixes rust-lang#122767.
    @nikic
    nikic committed Apr 8, 2024
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  2. force_array -> is_consecutive 

    The actual ABI implication here is that in some cases the values
are required to be "consecutive", i.e. must either all be passed
in registers or all on stack (without padding).

Adjust the code to either use Uniform::new() or Uniform::consecutive()
depending on which behavior is needed.

Then, when lowering this in LLVM, skip the [1 x i128] to i128
simplification if is_consecutive is set. i128 is the only case
I'm aware of where this is problematic right now. If we find
other cases, we can extend this (either based on target information
or possibly just by not simplifying for is_consecutive entirely).
    @nikic
    nikic committed Apr 8, 2024
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