> allow merging them, which QEMU does, even if just at basic block level. IIRC post-SIGSEGV state is not 100% correct in QEMU

A quick and very limited testing shows that all registers and even status flags are correct in the ucontext in the signal handler. Glancing at the source code, TCG optimizations (e.g., liveness analysis) primarily apply to temporaries, but the architectural registers are always updated. That's also why QEMU is so slow (and easy to beat in papers, which very often disregard strict correctness).

Function-level lifting to LLVM gives massive performance improvements, but sacrifices correctness w.r.t. signals (synchronous and also asynchronous, unless you add a check for pending signals to every loop, or somehow else recover the state in not-too-distant time).