macOS top-half avx / avx2 registers aren't handled correctly

[milianw]

Vc version / revision	Operating System	Compiler & Version	Compiler Flags	Assembler & Version	CPU
1.4	macOS 10.15.3	Apple clang version 11.0.0 (clang-1100.0.33.17)	avx / avx2	Apple clang version 11.0.0 (clang-1100.0.33.17)	2,6GHz Dual-Core Intel Core i5 from MacBook Pro Retina, 13-inch, Mid 2014

In a project of ours that is using Vc 1.4 a colleague reported strange issues: The images we produce using Vc are striped, with the first four pixels looking good and then the following four pixels being black. Turns out, the code that's fine on most platforms and even on another macOS machine with a different CPU, behaves very wrong. I believe the existing test suite of Vc also shows that issue, see below.

Is this a macOS/clang bug, or something within Vc? Any way to workaround this?

Testcase

arithmetics_avx:
27:  FAIL: ┍ at /Users/kdab/Vc/tests/arithmetics.cpp:231 (0x106d4beb4)):
27:  FAIL: │ test * test - test ([16797700, 16797700, 16797700, 16797700, 16797700, 16797700, 16797700, 16797700]) ≈ Vec(j * j - j) ([16797702, 16797702, 16797702, 16797702, 16797702, 16797702, 16797702, 16797702]) -> m[0000 0000]
27:  FAIL: │ distance: [-1, -1, -1, -1, -3.35544e+07, -3.35544e+07, -3.35544e+07, -3.35544e+07] ulp, allowed distance: ±1 ulp 
27:  FAIL: ┕ testMulSub<simd< float, AVX>>

arithmetics_avx2:
28:  FAIL: ┍ at /Users/kdab/Vc/tests/arithmetics.cpp:231 (0x10ca05724)):
28:  FAIL: │ test * test - test ([16797700, 16797700, 16797700, 16797700, 16797700, 16797700, 16797700, 16797700]) ≈ Vec(j * j - j) ([16797702, 16797702, 16797702, 16797702, 16797702, 16797702, 16797702, 16797702]) -> m[0000 0000]
28:  FAIL: │ distance: [-1, -1, -1, -1, -3.35544e+07, -3.35544e+07, -3.35544e+07, -3.35544e+07] ulp, allowed distance: ±1 ulp 
28:  FAIL: ┕ testMulSub<simd< float, AVX>>

Actual Results

The following tests FAILED:
	 27 - arithmetics_avx (Failed)
	 28 - arithmetics_avx2 (Failed)
	 59 - ulp_avx (Failed)
	 60 - ulp_avx2 (Failed)
	 71 - math_avx (Failed)
	 72 - math_avx2 (Failed)
	103 - gatherinterleavedmemory_avx (Failed)
	104 - gatherinterleavedmemory_avx2 (Failed)```

Expected Results

no failures

[0x6e]

Vc version / revision	Operating System	Compiler & Version	Compiler Flags	Assembler & Version	CPU
1.4	macOS 10.15.3	Apple clang version 11.0.0 (clang-1100.0.33.17)	avx / avx2	Apple clang version 11.0.0 (clang-1100.0.33.17)	4.01 GHz Quad-Core Intel Core i7 6700K

Here is the complete test run output from my failing machine: test-mac.log

[0x6e]

My MacBook Pro has no issues, all tests pass. Note the software stack is the same:

Vc version / revision	Operating System	Compiler & Version	Compiler Flags	Assembler & Version	CPU
1.4	macOS 10.15.3	Apple clang version 11.0.0 (clang-1100.0.33.17)	avx / avx2	Apple clang version 11.0.0 (clang-1100.0.33.17)	MacBook Pro (Retina, 15-inch, Mid 2015) 2.5 GHz Intel Core i7

test-mbp.log

[0x6e]

Note that on my failing machine, if I use gcc all tests pass:

Vc version / revision	Operating System	Compiler & Version	Compiler Flags	Assembler & Version	CPU
1.4	macOS 10.15.3	g++-9 (Homebrew GCC 9.2.0_3) 9.2.0	avx / avx2	g++-9 (Homebrew GCC 9.2.0_3) 9.2.0	4.01 GHz Quad-Core Intel Core i7 6700K