[llvm-commits] [ARM] Fix for blocking PR13790. AAPCS byval params issue.

[Stepan Dyatkovskiy]

Hi Anton,

 > How will you patch influence APCS?

To make all clear, I updated patch and made it fixing AAPCS case only.

I can't get codegen to emit 8 bytes aligned structure parameter, and I 
can't get to use 8-bytes aligned stack. ARM target ignores 
--stack-alignment option.

But if some people can get llc to use 8 bytes alignment for APCS, I'm 
90% sure that this issue fires again.

Relative to APCS:

In APCS we can found the next statement:
"The APCS does not define:
- the layout in store of records, arrays, as so forth, used by 
ARM-targeted compilers
for C and C++
..."
and a bit below:
"This means that, the mapping from language-level data types and 
arguments to APCS
words is defined by each language implementation, not by the APCS."
(APCS, 9.2.7 Data representation and argument passing)

APCS was defined in ARM Software Development Toolkit, ARM DUI 0041C.

In this document I also found next recommendation for compiler developers:
"The alignment of a non-packed structure is the larger of:
- The maximum alignment required by any of its fields
... "
(3.2.4 Structured data types)

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