Use ldc1 and sdc1 for the prologue and epilogue on LOONGSON3A

[chenx97]

This fix is similar to
2d80641.

[chenx97]

I first ran into this bug in CTEST(potrf, smoketest_trivial), where a constant like 1e-5, with underlying bits 0x3ee4f8b588e368f1, can become 1.1346735896e-314 because of the lost upper 32 bits, making the number significantly smaller than the actual error produced by the smoke test and leads to test failure.

I don't plan to add LOONGSON3A to the mips64 CI workflow because qemu doesn't work with this test while my Loongson 3B4000 does...

[martin-frbg]

so this is the real solution for what I papered over in #4810 ?

[chenx97]

so this is the real solution for what I papered over in #4810 ?

My fix is for the old mips64el loongson. I haven't checked loongarch64 yet.

[chenx97]

TEST 99/103 potrf:smoketest_trivial [FAIL]
  ERR: test_potrs.c:535  L s(0,0) difference: 1.19209e-07

I see. This is exactly what I saw when the test was failing for the 3B4000. Maybe you can give it a try.

[chenx97]

    PROLOGUE

    addi.d     $sp,    $sp,   -128
    SDARG      $r23,   $sp,   0
    SDARG      $r24,   $sp,   8
    SDARG      $r25,   $sp,   16
    SDARG      $r26,   $sp,   24
    SDARG      $r27,   $sp,   32
    ST         $f23,   $sp,   40
    ST         $f24,   $sp,   48
    ST         $f25,   $sp,   56
    ST         $f26,   $sp,   64
    ST         $f27,   $sp,   72
    ST         $f28,   $sp,   80
    ST         $f29,   $sp,   88
    ST         $f30,   $sp,   96
    ST         $f31,   $sp,   104
    ST         ALPHA_R,$sp,   112
    ST         ALPHA_I,$sp,   120

oof...

[martin-frbg]

I see @fengrl tried to explain this to (a younger) me six years ago but I did not understand :(
Indeed looks like the loongarch64 kernel development followed the same scheme, people getting tricked by the LD/ST macro "sometimes" expanding to the correct fld.d/fst.d instruction.
The spurious error in the potrs test appears to be gone after updating the LOONGSON3R5 gemm kernels but I need to do some more testing before I can put that into a PR...