It's always something

[Walter Bright]

I just spent about 3 hours tracking down the strangest problem. Win64 exception 
handling would work fine, but when I'd turn on -g, it would crash.

At first I thought "I'm generating a bad object file with -g." But since dmd 
isn't emitting any Win64 symbolic debug info, a check shows the object files are 
the same with and without -g.

So it must be something with linking? I spent a lot of time going over and over 
the relocation fixups (could there be a missing offset?), but could find nothing 
wrong.

Instrumenting the Phobos eh handler code, it seems my handler table was pointing 
off into la-la land when linking with /DEBUG. What the hell?

I thought I'd dump the first byte of where it was pointing to, and look in the 
assembler output and see if that byte was at a predictable offset from where it 
was supposed to be. The actual function started with C3, but the handler table 
pointed to E9. There was no E9 in the object file. What the hell?

Finally, it dawned on me. E9 is a JMP instruction! For some reason, the 
Microsoft linker inserts a bunch of trampolines when linking for debug.

The fix, then, was for the eh handler to look and see if the handler table is 
pointing to an E9, and if so, then adjust the function address to be where ever 
the JMP goes.

And so it goes...