Friday, January 30, 2009

Quiz answer

Reader Mitch (not my advisor, apparently) comes closest to the answer to my quiz question:
Some kind of alignment thing? I can't think of how code would be affected by that, though.
As it happens, changing the code was affecting the particular size of the activation record allocated by the GCC-generated code. Somehow the extra code ended up with a slightly smaller activation record. And then it turned out that the slight difference (something like 8 or 16 bytes) was pushing the activation record over the edge of a page boundary. Boom!

It's still a mystery to me exactly how throwing in an asm("noop\n") can affect the size of the activation record. Such are the arcane mysteries of GCC code generation. I had to catch my flight home before Dave had worked out all the details but by now I bet he has an even better understanding of it. It was fun and inspiring to watch Dave's detective skills in action.

1 comment:

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