Question on arithmetic overflow detection support in C2

[Vladimir Kozlov]

It will be difficult. Almost all data nodes produce only one result and this 
rule is used in all parts of C2 (from parser to RA). One exception is 
conditional stores which produce flag and memory state (look in macro.cpp in 
expand_allocate_common() method). This case handled specially everywhere. And 
you do need to add overflow and non-overflow pair to BoolTest values.

Vladimir

Krystal Mok wrote:
> Hi all,
> 
> I'm doing some experiments in C2, trying to add support in the IR for 
> arithmetic overflow detection. Looks like it's quite involved, though.
> Any suggestions on how it could be done would be greatly appreciated :-)
> 
> My questions:
> 
> 1. The structure of IfNode: does the IR have to be in the form of
> If_proj->If->Bool->Cmp
> 
> Although there's code in idealize_test() (in ifnode.cpp) that checks if 
> iff->in(1)->is_Bool(), other places seem to assume the sole data input 
> to If is a Bool, e.g. the transformation in Matcher::find_shared().
> 
> What if I wanted to add some node type other than Bool as the test input 
> to If? Something like (If (CheckOverflow expr)), does that feel right?
> 
> 2. The BoolTest kinds, does it feel right to add a pair of kinds for 
> overflow? Like BoolTest::ovf and BoolTest::noovf. They'll still fit in 
> the 3-bit encoding.
> 
> If this pair is added, then to make it useful, the input to Bool 
> wouldn't always be Cmp anymore; instead it could be any arithmetic nodes 
> that modifies the condition flags as a side-effect.
> And...that doesn't seem to fit in C2's IR right now, since the only kind 
> of node that models DEF of the condition flags are the Cmp nodes.
> 
> 3. As a quick-n-dirty experiment, I tried not to touch the 
> machine-independent IR, and instead modify only the ad file to pattern 
> match the current Java implementation of Match.addExact(int, int) to 
> make it emit a jo (jump-if-overflow) instruction for overflow detection, 
> but it failed to compile.
> The patch is here: https://gist.github.com/3818aa3acbc78e28e7fa
> 
> (Aside: the Ideal graph of the overflow check expression is like this:
> http://dl.iteye.com/upload/picture/pic/114476/674e0a78-5515-3da3-8ea6-6eedbeb32f8e.png
> This is the state right before matching the matcher rules)
> 
> ADLC didn't complain, but the generated jmpAddOverflowNode::Expand() 
> function had compilation errors, saying:
> 
> ../generated/adfiles/ad_x86_64_expand.cpp: In member function ‘virtual 
> MachNode* jmpAddOverflowNode::Expand(State*, Node_List&, Node*)’:
> ../generated/adfiles/ad_x86_64_expand.cpp:5915: error: ‘num8’ was not 
> declared in this scope
> 
> Apparently I was doing something wrong in the ad file, but I don't quite 
> get what it was.
> 
> Regards,
> Kris