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<p style="margin: 0px 0px 1.2em !important;">Hi,<br>
thanks for the report.</p>
<p style="margin: 0px 0px 1.2em !important;">By reading the
Aarch64 ABI:</p>
<p style="margin: 0px 0px 1.2em !important;"></p>
<div class="markdown-here-exclude">
<p></p>
<blockquote type="cite">5.9.5 Homogeneous Aggregates<br>
A Homogeneous Aggregate is a composite type where all of the
Fundamental Data Types of the members<br>
that compose the type are the same. The test for homogeneity
is applied after data layout is completed and<br>
without regard to access control or other source language
restrictions. Note that for short-vector types the<br>
fundamental types are 64-bit vector and 128-bit vector; the
type of the elements in the short vector does not<br>
form part of the test for homogeneity.<br>
A Homogeneous Aggregate has a Base Type, which is the
Fundamental Data Type of each Member. The<br>
overall size is the size of the Base Type multiplied by the
number uniquely addressable Members; its<br>
alignment will be the alignment of the Base Type.<br>
5.9.5.1 Homogeneous Floating-point Aggregates (HFA)<br>
A Homogeneous Floating-point Aggregate (HFA) is a Homogeneous
Aggregate with a Fundamental Data<br>
Type that is a Floating-Point type and at most four uniquely
addressable members.<br>
5.9.5.2 Homogeneous Short-Vector Aggregates (HVA)<br>
A Homogeneous Short-Vector Aggregate (HVA) is a Homogeneous
Aggregate with a Fundamental Data<br>
Type that is a Short-Vector type and at most four uniquely
addressable members.</blockquote>
<p></p>
</div>
<p style="margin: 0px 0px 1.2em !important;"></p>
<p style="margin: 0px 0px 1.2em !important;">I’m frankly not clear
at all at what should happen in some of the cases you have in
your examples. For instance, this:</p>
<pre style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;font-size: 1em; line-height: 1.2em;margin: 1.2em 0px;"><code style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;white-space: pre; overflow: auto; border-radius: 3px; border: 1px solid rgb(204, 204, 204); padding: 0.5em 0.7em; display: block !important;">// HFA
typedef union {
struct {
struct {
int x;
} arr1[0];
};
float arr2[3];
} T22;
</code></pre>
<p style="margin: 0px 0px 1.2em !important;">Works as an HFA, I
suppose, because “x” is not an addressable member. But then…</p>
<pre style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;font-size: 1em; line-height: 1.2em;margin: 1.2em 0px;"><code style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;white-space: pre; overflow: auto; border-radius: 3px; border: 1px solid rgb(204, 204, 204); padding: 0.5em 0.7em; display: block !important;">// not HFA, int registers
typedef union {
struct {
struct {
long long x;
} arr1[0];
};
float arr2[3];
} T23;
</code></pre>
<p style="margin: 0px 0px 1.2em !important;">This also only has
one “addressable member” (whatever that means). But clang uses
float registers in the latter, not the former.</p>
<p style="margin: 0px 0px 1.2em !important;">Also, switching
godbolt to use GCC I don’t see any difference in the assembly
generated for the two examples. They both seem to use int
registers (e.g. mov, not fmov).</p>
<p style="margin: 0px 0px 1.2em !important;">So, there seems to be
some issue here, although, given the very vague definition of
HFA in the ABI, it is not clear to me whether we’re staring at a
FFM bug, an llvm bug, or a gcc bug.</p>
<p style="margin: 0px 0px 1.2em !important;">The FFM code seems to
be very conservative: if there’s a member layout in the
struct/union that’s not a value layout (padding, struct/union,
sequence), then the struct/union is not an HFA. It is possible
that’s too restrictive, but we’d need some authoritative opinion
on how the rules have to be interpreted (cc’ing Nick).</p>
<p style="margin: 0px 0px 1.2em !important;">Cheers<br>
Maurizio</p>
<p style="margin: 0px 0px 1.2em !important;">On 06/10/2024 10:16,
Владимир Козелков wrote:</p>
<p style="margin: 0px 0px 1.2em !important;"></p>
<div class="markdown-here-exclude">
<p></p>
<blockquote type="cite" cite="mid:CAGF0iuRV1_ZUMEb4FRPM=AJpPKfdCQTJECs+z4qj_d7rKf37Vg@mail.gmail.com">
<div dir="ltr">Hello! <br>
<div>Under the <a href="https://github.com/openjdk/jdk/pull/21041" moz-do-not-send="true">pull request</a> I was advised to
write about new issues to this email.<br>
</div>
<div><br>
</div>
<div>I'm building my own port of the Panama project to
Android, which uses LibLLVM for native code generation, so
I've been doing a lot of research into how the calling
convention works there.<br>
</div>
<div><br>
</div>
<div>Based on the data I received, I created my own <a href="https://github.com/vova7878/PanamaPort/blob/master/AndroidPanama/src/main/java/com/v7878/foreign/_LLVMCallingConvention.java" moz-do-not-send="true">classifier</a> of structures for
the arranger with an eye on LLVM (In most cases, it
calculates registers itself)<br>
</div>
<div><br>
</div>
<div>Initially my implementation rejected structures and
sequences of size 0, but then I added support for them
back. Everything worked pretty well until I dug into the
godbolt tests and compared my implementation to the
reference implementation from OpenJDK.</div>
<div><br>
</div>
<div>It seems to me that<a href="https://github.com/openjdk/jdk/blob/260d4658aefe370d8994574c20057de07fd6f197/src/java.base/share/classes/jdk/internal/foreign/abi/aarch64/TypeClass.java#L85" moz-do-not-send="true">
jdk.internal.foreign.abi.aarch64.TypeClass.isHomogeneousFloatAggregate</a>
does not work correctly for union types and sequences with
nested GroupLayout. Additionally, I made examples in
godbolt that show how it determines HFA<br>
</div>
<div><br>
</div>
<div>C++ <a href="https://godbolt.org/z/fcsMvaKhd" moz-do-not-send="true" class="moz-txt-link-freetext">https://godbolt.org/z/fcsMvaKhd</a></div>
<div>C <a href="https://godbolt.org/z/er8Yev7ME" moz-do-not-send="true" class="moz-txt-link-freetext">https://godbolt.org/z/er8Yev7ME</a></div>
<div><br>
</div>
<div>I don't have a PC on aarch64 to run tests on java, but
it looks to me like OpenJDK implementation is not correct
for some cases<br>
</div>
</div>
</blockquote>
<p></p>
</div>
<p style="margin: 0px 0px 1.2em !important;"></p>
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