Assertion failure with decomposed record pair

[rmacnak-google]

The tests

language/records/simple/constants_and_field_access_test/1 Crash (expected Pass)

are failing on configurations

vm-aot-win-debug-x64c

stderr:
../../runtime/vm/compiler/backend/il_x64.cc: 411: error: expected: result_lo == CallingConventions::kReturnReg
version=3.0.0-edge.262efd84fd2b14cf64dfa6c937856d717fbe6e1e (be) (Fri Feb 24 04:09:12 2023 +0000) on "windows_x64"
pid=7376, thread=3548, isolate_group=isolate(00000276633EFFA0), isolate=(nil)(0000000000000000)
os=windows, arch=x64, comp=yes, sim=no
isolate_instructions=0, vm_instructions=0
fp=7b1cefcf10, sp=7b1cefcf10, pc=7ff67865e6d2
  pc 0x00007ff67865e6d2 fp 0x0000007b1cefcf10 Dart_RunLoop+0x444700
-- End of DumpStackTrace
=== Crash occurred when compiling file:///C:/b/s/w/ir/tests/language/records/simple/constants_and_field_access_test.dart_::_getP1N1 in AOT mode in GenerateCode pass
=== When compiling block B1[function entry]:2
ParallelMove ? <- ?
=== When compiling instruction Return:16(v9 T{_Record})
*** BEGIN CFG
GenerateCode
==== file:///C:/b/s/w/ir/tests/language/records/simple/constants_and_field_access_test.dart_::_getP1N1 (RegularFunction)
B0[graph]:0 {
      v5 <- UnboxedConstant(#40 int64) [40, 40] T{_Smi}
}
B1[function entry]:2
ParallelMove ? <- ?
    CheckStackOverflow:8(stack=0, loop=0)
    MoveArgument(v5, SP+0)
    v6 <- StaticCall:30( getP1N1Rec<0> v5, result_type = T{_Record}) T{_Record}
    v7 <- ExtractNthOutput(Extract 0 from v6) T{*?}
    v8 <- ExtractNthOutput(Extract 1 from v6) T{*?}
    v9 <- MakePair(v7, v8) T{*?}
    Return:16(v9 T{_Record})
*** END CFG

--- Re-run this test:

@alexmarkov

[alexmarkov]

Reproduced the crash (on Windows but not Linux) and investigating it.
It looks like locations change during code generation:

After AllocateRegisters:

 10:     v31 <- ExtractNthOutput(Extract 0 from v26) T{*?} ((rax, rdx)) => rcx
 12:     v32 <- ExtractNthOutput(Extract 1 from v26) T{*?} ((rax, rdx)) => rbx
 13:     ParallelMove rax <- rcx, rdx <- rbx
 14:     v36 <- MakePair(v31, v32) T{*?} (rax, rdx) => (rax, rdx)
 16:     Return:16(v36 T{_Record}) ((rax, rdx))

This looks correct.

When generating code for Return instruction:

    v7 <- ExtractNthOutput(Extract 0 from v6) T{*?} ((rdx, rax)) => rcx
    v8 <- ExtractNthOutput(Extract 1 from v6) T{*?} ((rdx, rax)) => rbx
    v9 <- MakePair(v7, v8) T{*?} (rax, rdx) => (rdx, rax)
    Return:16(v9 T{_Record}) ((rdx, rax))

rax and rdx are swapped in the PairLocation in MakePair output and Return input.