On Wed, Mar 20, 2019 at 10:13:23AM +0000, Justin Paston-Cooper wrote: > Section 6.2.5.2 outlines the line number information state machine's > opcodes. One of them is "DW_LNS_set_epilogue_begin". Its definition > is: > > ----- > The DW_LNS_set_epilogue_begin opcode takes no operands. It sets the > epilogue_begin register to “true”. When a breakpoint is set on the > exit of a function or execution steps over the last executable > statement of a function, it is generally desirable to suspend > execution after completion of the last statement but prior to tearing > down the frame (so that local variables can still be examined). > Debuggers generally cannot properly determine where this point is. > This command allows a compiler to communicate the location(s) to use. > Note that the function to which the epilogue end applies cannot be > directly determined from the line number information alone; it must be > determined in combination with the subroutine information entries of > the compilation (including inlined subroutines). In the case of a > trivial function, both prologue end and epilogue begin may occur at > the same address. > -----
How should this work with shrink-wrapping? The whole point of that is you do not tear down the frame after all other code, etc. I don't see how we can do better than putting this DW_LNS_set_epilogue_begin right before the actual return -- and that is after all the tear down etc. > I had a look into > gcc's involvement with "epilogue_begin". The pass called > "pass_thread_prologue_and_epilogue" transitively calls a function > defined in function.c called "make_epilogue_seq", which as far as I > understand emits an "epilogue_begin" note. I'm not sure how this pass > ties in with each frontend. Does a frontend need to support this pass > explicitly? Nope. This pass is very late in the backend, after register allocation and everything. A frontend will just make functions "return", and that's all it has to do. Segher
