If every single SIGTRAP was caused by an INT3 instruction, then this would work, but SIGTRAP can also be generated by single stepping where subtracting 1 would be incorrect.

GDB already compensates for breakpoints it inserts itself because it knows their locations, so it subtracts 1 as you suggest. It could also recognize embedded INT3 instructions but that requires distinguishing their traps from other causes of SIGTRAP which is much more complicated. So instead GDB generally leaves the instruction pointer unchanged for embedded INT3 instructions.

Here's a discussion of precisely this issue from the GDB developers:

https://sourceware.org/pipermail/gdb/2009-February/034621.ht...