[aarch64-port-dev ] RFR: 8075930: AARCH64: Use FP Register in C2

[Edward Nevill]

Hi Zoltán,

Thanks for the review. I have updated the patch to take account of your comments. New patch at

http://cr.openjdk.java.net/~enevill/8075930/webrev.02/

Could someone please review this patch and if approved push.

See inline comments below.

On Wed, 2015-03-25 at 14:28 +0100, Zoltán Majó wrote:
> On 03/25/2015 02:02 PM, Edward Nevill wrote:
> > Hi,
> >
> > The aarch64 C2 compiler makes no use of the FP register even though this register is saved on entry and restored on exit and is therefore always available for allocation.
> 
> It seems to me that the C2 compiler currently saves the SP into FP on 
> method handle invokes. This functionality is encoded by 
> aarch64_enc_java_handle_call in aarch64.ad. C1 does the same (see usages 
> of FrameMap::method_handle_invoke_SP_save_opr()).

So, this is an issue for C2 only, since my proposed patch does not affect register usage in C1, so C1 will continue to keep FP as unallocated.

The change required for C2 is to let the register allocator know that FP will be killed by a methodhandle call. This is accomplished by the following change in aarch64.ad

 const RegMask Matcher::method_handle_invoke_SP_save_mask() {
-  return RegMask();
+  return FP_REG_mask();
 }

> 
> Stack walking relies on having the SP saved into FP for method handle 
> invokes (e.g., in frame::adjust_unextended_sp()).
> 
> Saving the SP into FP is not needed on some architectures (e.g., x86). 
> I'm currently working on an enhancement that disables saving the SP into 
> FP on x86 (see JDK-8068945). Maybe that could work for aarch64 as well.
> 
> Does your change consider this aspect? Maybe running the tests in 
> jdk/test/java/lang/invoke/ could trigger some failures that are caused 
> by the FP not containing the SP of the caller of a method handle invocation.
> 

I have run the jdk/test/java/lang/invoke tests both with and without the
proposed patch. In bot cases 62 tests out of 62 passed.

I have also rerun JTreg hotspot and langtools with the following
results:-

Before:-

Hotspot: Test results: passed: 596; failed: 190; error: 26
Langtools: Test results: passed: 3,237; error: 9

After:-

Hotspot: Test results: passed: 596; failed: 190; error: 26
Langtools: Test results: passed: 3,238; error: 8

All the best,
Ed.