[llvm-dev] LLVM struct, alloca, SROA and the entry basic block

[Benoit Belley via llvm-dev]

Hi everyone,

We have noticed that the SROA pass will only eliminate 'alloca' instructions if those are located in the entry basic block of a function.

As a general recommendation, should the LLVM IR emitted by our compiler always place 'alloca' instructions in the entry basic block ? (I couldn't find any recommendations concerning this matter.)

In addition, we have noticed that the MemCpy pass will attempt to copy LLVM struct using moves that are as large as possible. For example, a struct of 3 floats is copied using a 64-bit and a 32-bit move. It is therefore important that such a struct be aligned on 8-byte boundary, not just 4 bytes! Else, one runs the risk of triggering store-forwarding failure pipelining stalls (which we did encountered really badly with one of our internal performance benchmark).

Is there any guidelines for specifying the alignment of LLVM structs allocated by alloca instructions ? Is rounding down to the structure size to the next power of 2 a good strategy ? Will the MemCpy pass issue moves of up to 64-bytes on AVX-512 capable processors ?

Cheers,
Benoit

Benoit Belley
Sr Principal Developer
M&E-Product Development Group

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