[9] RFR(S): 8148490: RegisterSaver::restore_live_registers() fails to restore xmm registers on 32 bit

[Vladimir Kozlov]

Tobias, please verify that 64-bit code works correctly.
About 32-bit code.

Please verify correctness of next asserts:

      assert(UseAVX > 0, "512bit vectors are supported only with EVEX");
      assert(MaxVectorSize == 64, "only 512bit vectors are supported now");

Originally we could have vectors even with only 64bit XMM registers. MaxVectorSize and UseAVX can be set on command line 
- what happens in such case? No vectorization?

May be it is done because we save whole 128bit XMM always. Still MaxVectorSize == 64 condition is strange.

Thanks,
Vladimir

On 1/29/16 6:16 AM, Tobias Hartmann wrote:
> Hi,
>
> please review the following patch:
> https://bugs.openjdk.java.net/browse/JDK-8148490
> http://cr.openjdk.java.net/~thartmann/8148490/webrev.00/
>
> RegisterSaver::save_live_registers() and RegisterSaver::restore_live_registers() are used by the safepoint handling code to save and restore registers. The following code is emitted to save and restore XMM/YMM registers on 32 bit:
>
> Save:
>     ...
>     0xf34ca12e:	vmovdqu %xmm0,0xb0(%esp)
>     0xf34ca137:	vmovdqu %xmm1,0xc0(%esp)
>     ...
>     0xf34ca16d:	vmovdqu %xmm7,0x120(%esp)
>     0xf34ca176:	sub    $0x80,%esp
>     0xf34ca17c:	vextractf128 $0x1,%ymm0,(%esp)
>     0xf34ca183:	vextractf128 $0x1,%ymm1,0x10(%esp)
>     ...
>     0xf34ca1b3:	vextractf128 $0x1,%ymm7,0x70(%esp)
>     ...
>
> Restore:
>     ...
>     0xf34ca202:	vinsertf128 $0x1,(%esp),%ymm0,%ymm0
>     0xf34ca209:	vinsertf128 $0x1,0x10(%esp),%ymm1,%ymm1
>     ...
>     0xf34ca239:	vinsertf128 $0x1,0x70(%esp),%ymm7,%ymm7
>     0xf34ca241:	add    $0x80,%esp
>     0xf34ca247:	vmovdqu 0x130(%esp),%xmm0
>     0xf34ca250:	vmovdqu 0x140(%esp),%xmm1
>     ...
>     0xf34ca286:	vmovdqu 0x1a0(%esp),%xmm7
>     ...
>
> The stack offsets for the vmovdqu instructions are wrong, causing the XMM registers to contain random values after a safepoint. The problem is that "additional_frame_bytes" is added to the stack offset although the stack pointer is incremented just before:
>
> 283     __ addptr(rsp, additional_frame_bytes); // Save upper half of YMM registers
>
> The regression test fails with "Test failed: array[0] = 1973.0 but should be 10.000" because the vectorized loop returns a wrong result.
>
> I spotted and fixed the following other problems:
> - the vmovdqu instructions should be emitted before restoring YMM and ZMM because they zero the upper part of the XMM registers (i.e. YMM/ZMM)
> - if 'UseAVX > 2' is set/available, we save the ZMM registers as well but we do not increment 'additional_frame_words' accordingly (we need another 8*32 bytes of stack space)
>
> Unfortunately, I don't have access to a CPU with the AVX-512 instruction set to test the "UseAVX > 2" related changes. Michael, could you verify the changes?
>
> The problems were introduced by the fix for JDK-8142980.
>
> Thanks,
> Tobias
>