Question on arithmetic overflow detection support in C2

Krystal Mok rednaxelafx at
Mon Jun 18 19:54:17 PDT 2012

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

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:

(Aside: the Ideal graph of the overflow check expression is like this:
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.

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