Question on arithmetic overflow detection support in C2 (original) (raw)

Krystal Mok rednaxelafx at gmail.com
Wed Jun 20 22:04:26 PDT 2012

Got the jo instruction emitted now. Thanks Vladimir!

I added a new FlagsProjNode type that derives from ProjNode, and explicitly overrides bottom_type() and ideal_reg() to TypeInt::CC and Op_RegFlags, respectively. That enabled the projection node matching to work.

Current status of the patch is avaiable here: https://gist.github.com/b24382a3dda090ab63ff

But there are still some unexpected behavior. I was expecting the Bool(... [ov]) node to be matched into a cmpOp, and the If node to be matched into a jmpCon; instead, they're actually matched into the unsigned version. Can't figure out what was going on yet.

The IR graph of Math.addExact(int, int),

after Optimize(): http://dl.iteye.com/upload/picture/pic/114606/0e8e47af-729e-36ca-baee-ed15174dd178.png

before matching: http://dl.iteye.com/upload/picture/pic/114610/05e81e91-ef99-3d9c-a543-12dacb9c2ae4.png (so far so good)

after matching and GCM: http://dl.iteye.com/upload/picture/pic/114612/da53b8cf-3ed2-30d2-9cfa-a1a43e94a4c2.png (oops...got jmpConU instead of the expected jmpCon)

Another problem is that I had to use a fixed register for the "dst" of checkedAddI_reg and its data result projection. In this case I'm using eax; but any fixed RegI will do.

The cause is stated before: I don't know how to specify the relationship between the "dst" of checkedAddI_reg and its data result projection, so as to make them use the same register.

If I can fix these two problems, then I'll go back and make a full-fledged intrinsic for Math.addExact(int, int). The way I pattern match the overflow checking logic wouldn't work when Math.addExact(int, int) is inlined into a caller with complex control-flow.

Thanks, Kris

On Thu, Jun 21, 2012 at 10:07 AM, Vladimir Kozlov < vladimir.kozlov at oracle.com> wrote:

C2 matchs from bottom to top of graph. So first it tries to match If node and failed. In second mail I explained that State for Proj node is incorrect for your case because idealreg() is incorrect:

s->DFA( m->idealreg(), m ); Vladimir -------------- next part -------------- An HTML attachment was scrubbed... URL: http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/attachments/20120621/162223cb/attachment.html

More information about the hotspot-compiler-dev mailing list