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I found it! With optimizations on GCC uses the eflags in certain cases to avoid doing extra condition branches. My assembly incorrectly cleared the zero flag in a place where it shouldn't have, causing it to go down the wrong branch, causing all sorts of bad things. All fixed 😁
Specifically GCC turned what was a simple condition jump into a test + cmovle instruction pair. Making the issue non obvious because execution of cmovle is based upon the state of the eflags
I've been writing a complex mutation engine that dynamically modifies compiled C++ code. Now there's alot of assembly involved, but I got it to work. I finished off writing the last unit test before it was time to port it all to windows. I switched into a release build, ready to bask in the glory of it all. FUCKING GCC OPTIMIZATIONS BROKE EVERYTHING. I had been doing all my dev in debug mode and now some obscure optimization GCC does in release mode is causing a segfault...somewhere. Just when I thought I was done 😅
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