On i386, if we call __fentry__ immediately on entry the return address will be in 4(%esp), so I fail to see how you could not reliably have the return address. Other arches would have different constraints, of course.
"Frederic Weisbecker" <fweis...@gmail.com> wrote: >On Thu, Nov 19, 2009 at 03:05:41PM -0500, Steven Rostedt wrote: >> On Thu, 2009-11-19 at 20:46 +0100, Frederic Weisbecker wrote: >> > On Thu, Nov 19, 2009 at 02:28:06PM -0500, Steven Rostedt wrote: >> >> > > <function>: >> > > call __fentry__ >> > > [...] >> > > >> > > >> > > -- Steve >> > >> > >> > I would really like this. So that we can forget about other possible >> > further suprises due to sophisticated function prologues beeing before >> > the mcount call. >> > >> > And I guess that would fix it in every archs. >> >> Well, other archs use a register to store the return address. But it >> would also be easy to do (pseudo arch assembly): >> >> <function>: >> mov lr, (%sp) >> add 8, %sp >> blr __fentry__ >> sub 8, %sp >> mov (%sp), lr >> >> >> That way the lr would have the current function, and the parent would >> still be at 8(%sp) >> > > >Yeah right, we need at least such very tiny prologue for >archs that store return addresses in a reg. > > >> > >> > That said, Linus had a good point about the fact there might other uses >> > of mcount even more tricky than what does the function graph tracer, >> > outside the kernel, and those may depend on the strict ABI assumption >> > that 4(ebp) is always the _real_ return address, and that through all >> > the previous stack call. This is even a concern that extrapolates the >> > single mcount case. >> >> As I am proposing a new call. This means that mcount stay as is for >> legacy reasons. Yes I know there exists the -finstrument-functions but >> that adds way too much bloat to the code. One single call to the >> profiler is all I want. > > >Sure, the purpose is not to change the existing -mcount thing. >What I meant is that we could have -mcount and -real-ra-before-fp >at the same time to guarantee fp + 4 is really what we want while >using -mcount. > >The __fentry__ idea is more neat, but the guarantee of a real pointer >to the return address is still something that lacks. > > >> > >> > So I wonder that actually the real problem is the lack of something that >> > could provide this guarantee. We may need a -real-ra-before-fp (yeah >> > I suck in naming). >> >> Don't worry, so do the C compiler folks, I mean, come on "mcount"? > > >I guess it has been first created for the single purpose of counting >specific functions but then it has been used for wider, unpredicted uses :) > -- Sent from my mobile phone. Please excuse any lack of formatting.
