aarch64 AD-file / matching rule

[Andrew Haley]

On 04/23/2015 02:41 PM, Benedikt Wedenik wrote:
> One of the patterns I want to optimise is the following:
> 
> 0x0000007f8c2961b4: and	w2, w2, #0x7ffff8 0x0000007f8c2961b8: cmp	w2, #0x0 0x0000007f8c2961bc: b.eq	0x0000007f8c2968f4
> 
> 
> Here I see an opportunity for ands, b.eq.

It would make more sense to use cbz.  In fact, I have no idea why cbz
is not being used here.  I can't see any point to this pattern.


> [-client -Xcomp -XX:-TieredCompilation] : Here the cmp for #0x0 only occurs about 3 times in the whole disassembly, instead of about 200 times without these flags.

This tells you that the cmp for #0x0 is C1, not C2.

> My question is now how to find out why the rule does not match / if the rule is correct and how to find the actual rule which emits the code of my desired pattern.

1.  Use a debug build.

2.  Turn off tiered compilation.  Look at a disassembly and see which
    method and which line emits the code above.  You haven't provdied
    enough context.

3.  Forget about optimizing C1.  It will have no effect on benchmarks.

Andrew.