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

Berg, Michael C michael.c.berg at intel.com
Mon Feb 1 18:50:29 UTC 2016

Tobias, since it works everywhere now, let us proceed.
The code is ok in its current form.


-----Original Message-----
From: Tobias Hartmann [mailto:tobias.hartmann at oracle.com] 
Sent: Monday, February 01, 2016 3:33 AM
To: Berg, Michael C; Vladimir Kozlov
Cc: hotspot-compiler-dev at openjdk.java.net
Subject: Re: [9] RFR(S): 8148490: RegisterSaver::restore_live_registers() fails to restore xmm registers on 32 bit

Thanks Vladimir and Michael, for the reviews.

Michael wrote (off-thread):
> Tobias, I might take the restore code a different direction.  Would it be ok if I handed you code back which has an offset based approach that makes the direction of restores no longer dependent upon the current stack?

I will wait for his changes and then send out a new RFR.


On 29.01.2016 23:28, Berg, Michael C wrote:
> Tobias/Vladimir:
> I would change the two asserts to in the 64bit code to make the check clear:
>       assert(UseAVX > 0, "up to 512bit vectors are supported with EVEX");
>       assert(MaxVectorSize <= 64, "up to 512bit vectors are supported 
> now");
> As for testing with the patch applied to hotspot on a current jdk(01-29-16):
> Windows sde 32-bit: skx - pass, also ran and passed part of 
> specjvm2008 Windows 32-bit: hsw - pass, also ran and passed all of 
> specjvm2008 Windows sde 64-bit: skx - pass, also ran and passed part 
> of specjvm2008 Windows 64-bit: hsw -pass, also ran and passed all of 
> specjvm2008 : caveat Linux on skx: 32-bit - pass, also ran and passed 
> all of specjvm2008 Linux on skx:64-bit - pass, also ran and passed all 
> of specjvm2008
> We should proceed with checkin in the changelist after the usual testing. 
> Note: The above tests were done with the asserts changed on windows 
> only. The 64bit changes are mostly cosmetic.  It's the change to the additional_frame_bytes that makes it correct, we used equivalent constants in the stack adjustment beforehand, they had not been mapped to the movdqu for the non-vector case for a few iterations on the file.  Early on I did have that code though.
> Caveat: xml.transform fails with the changelist and without, I checked this against a 12-21-15 built jdk which is 1 month old, so we have a new bug that is causing this app to fail as well (on windows for 64bit) on hsw.
> I checked recent jbs traffic, the occurrence does not appear to be tracked at this time.
> -Michael
> -----Original Message-----
> From: Vladimir Kozlov [mailto:vladimir.kozlov at oracle.com]
> Sent: Friday, January 29, 2016 11:40 AM
> To: hotspot-compiler-dev at openjdk.java.net
> Cc: Berg, Michael C
> Subject: Re: [9] RFR(S): 8148490: 
> RegisterSaver::restore_live_registers() fails to restore xmm registers 
> on 32 bit
> 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. 
>> - 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

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