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SubjectRe: [RFC] ARC: initial ftrace support
Hi,

ARC-gcc has two modes to call the mcount routines. When using elf32
configuration, the toolchain is set to use newlib mcount. When
configured for linux, gcc toolchain is using a library call to _mcall
(single underscore) having blink as input argument.
So, using the proper linux toolchain, your patch should work.

//C

On Thu, Apr 2, 2020 at 4:17 AM Vineet Gupta <Vineet.Gupta1@synopsys.com> wrote:
>
> +CC Claudiu
>
> On 3/27/20 10:10 AM, Steven Rostedt wrote:
> > On Fri, 27 Mar 2020 18:53:55 +0300
> > Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com> wrote:
>
> Maybe add a comment that gcc does the heavy lifting: I have following in glibc
>
> +/* this is very simple as gcc does all the heavy lifting at _mcount call site
> + * - sets up caller's blink in r0, so frompc is setup correctly
> + * - preserve argument registers for original call */
>
> >> +noinline void _mcount(unsigned long parent_ip)
> >> +{
> >> + unsigned long ip = (unsigned long)__builtin_return_address(0);
> >> +
> >> + if (unlikely(ftrace_trace_function != ftrace_stub))
> >> + ftrace_trace_function(ip - MCOUNT_INSN_SIZE, parent_ip,
> >> + NULL, NULL);
> >> +}
> >> +EXPORT_SYMBOL(_mcount);
> >
> > So, ARCv2 allows the _mcount code to be written in C? Nice!
>
> Yeah, the gcc backend for -pg was overhauled recently so it is a first class "lib
> call" meaning we get all the register save/restore for free as well as caller PC
> (blink) as explicit argument to _mcount
>
> void bar(int a, int b, int c) {
> printf("%d\n", a, b, c);
> }
>
> bar:
> push_s blink
> std.a r14,[sp,-8]
> push_s r13
> mov_s r14,r1
> mov_s r13,r0
> mov_s r0,blink
> bl.d @_mcount
> mov_s r15,r2
>
> mov_s r3,r15 <-- restore args for call
> mov_s r2,r14
> mov_s r1,r13
> mov_s r0,@.LC0
> ld blink,[sp,12]
> pop_s r13
> b.d @printf
> ldd.ab r14,[sp,12]
>
> @Eugeniy, this patch looks ok to me, but a word of caution. This won't work with
> elf32 toolchain which some of the build systems tend to use (Alexey ?)
>
> The above _mcount semantics is only implemented for the linux tool-chains.
> elf32-gcc generates "legacy" __mcount (2 underscores, blink not provided as arg)
> likely done by Claudiu to keep newlib stuff unchanged. Perhaps elf32 gcc can add a
> toggle to get new _mcount.
>
> And this is conditional to ARCv2 due to future ties into dynamic ftrace and
> instruction fudging etc ? We may have to revisit that for BE anyhow given such a
> customer lining up.
>
> -Vineet

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