john.brawn added a comment.
In D62152#1508979 <https://reviews.llvm.org/D62152#1508979>, @efriedma wrote:
> Please verify my understanding of the following is correct:
>
> 1. getTypeUnadjustedAlign() is currently only used to compute calling
> convention alignment for ARM and AArch64.
Yes, the only places it's called are AArch64ABIInfo::classifyArgumentType and
ARMABIInfo::classifyArgumentType.
> 2. Without this patch, we use the unadjusted alignment to pass arguments, but
> the adjusted alignment to compute the alignment for the corresponding va_arg
> calls.
Yes.
> 3. Without this patch, the unadjusted alignment for non-struct types is
> actually adjusted based on attributes on typedefs.
Only in va_arg. For example if you look at
typedef int aligned_int __attribute__((aligned(8)));
aligned_int called_fn(int a1, aligned_int a2) {
return a2;
}
aligned_int calling_fn() {
return called_fn(1,2);
}
a2 is passed in r1 by calling_fn, and read from it in called_fn (both before
and after this patch). However for
typedef int aligned_int __attribute__((aligned(8)));
aligned_int called_fn(int a1, ...) {
va_list ap;
va_start(ap, a1);
aligned_int a2 = va_arg(ap, aligned_int);
va_end(ap);
return a2;
}
aligned_int calling_fn() {
return called_fn(1,2);
}
a2 is still passed in r1 by calling_fn, but the current clang behaviour in
called_fn is that it pushes r1-r3 then loads the next 8-byte-aligned value,
which will be the pushed r2 value. This patch fixes this so that it doesn't
8-byte align the load address, and so gets the pushed r1.
> I'm not confident about changing the calling convention again at this point
> for non-struct types. I guess it's obscure enough that changing it is
> probably okay. But would you mind splitting it into a separate followup,
> with a better description of the impact?
This patch only changes the va_arg behaviour, and this whole thing with
unadjusted alignment is from the published pcs
(https://developer.arm.com/docs/ihi0042/latest/procedure-call-standard-for-the-arm-architecture-abi-2019q1-documentation
and
https://developer.arm.com/docs/ihi0055/latest/procedure-call-standard-for-the-arm-64-bit-architecture,
though it's called "natural alignment" there).
Repository:
rC Clang
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D62152/new/
https://reviews.llvm.org/D62152
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