[llvm-bugs] [Bug 29092] New: psABI inconsistency when passing vector registers?

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https://llvm.org/bugs/show_bug.cgi?id=29092

            Bug ID: 29092
           Summary: psABI inconsistency when passing vector registers?
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: wenzel.jakob at epfl.ch
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Clang trunk does not pass vector types through registers when they are
contained in an aggregate data type. Consider the following simple program:

------------------------------
C code:

#include <immintrin.h>

typedef struct __attribute__((aligned(16))) {
    __m128 x, y;
} Structure;


__m128 test1(__m128 x, __m128 y) { return y; }
__m128 test2(Structure t) { return t.y; }

------------------------------

Assembly:

$ clang++ test.cpp -msse4.2 -I include -std=c++14 -o - -S -fomit-frame-pointer
-O3

__Z5test1Dv4_fS_:
           vmovaps    %xmm1, %xmm0
           retq

__Z5test29Structure: 
           vmovaps    24(%rsp), %xmm0       <------ copy from memory :(
           retq

------------------------------

The psABI spec
(https://github.com/hjl-tools/x86-psABI/blob/master/low-level-sys-info.tex#L506)
has the following to say about this:

Each field of an object is classified *recursively* (emphasis added) so that
always two fields are considered. The resulting class is calculated according
to the classes of the fields in the eightbyte [...]. 
(a)-(e) [... cases which don't apply ...]
(f) Otherwise class SSE is used.


Intuitively (and according to the spec), I would have expected that "x" is
assigned to %xmm0 and "y" is assigned to %xmm1.

I'm wondering if I am misreading the ABI here, or if Clang is in violation?

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