RISC-V: Dynamic tracing support

[namhyung]

To implement entrire uftrace features, it should support (full) dynamic tracing either with compiler support or not. Specifically it'd support the following but we need to check the compiler implementations first:

• -mfentry which generates calls to __fentry__
• -mfentry with -mnop-mcount which would generate NOPs
• -fpatchable-function-entry which adds NOPs
• full dynamic tracing of normal binary

[honggyukim]

I can confirm that -fpatchable-function-entry is available in RISC-V.

$ gcc -fpatchable-function-entry=2 s-abc.c 
$ objdump -d a.out | grep -A 5 "<main>:$"
000000000000076e <main>:
 76e:   0001                    nop
 770:   0001                    nop
 772:   7179                    addi    sp,sp,-48
 774:   f406                    sd      ra,40(sp)
 776:   f022                    sd      s0,32(sp)

But -mfentry and -mnop-mcount are not available just like ARM/AArch64. I think that's only available in i386 and x86_64.

$ gcc -mfenty -mnop-mcount s-abc.c 
gcc: error: unrecognized command-line option '-mfenty'
gcc: error: unrecognized command-line option '-mnop-mcount'

Here is gcc version info.

$ gcc -v 2>&1 | grep Target
Target: riscv64-linux-gnu

$ gcc --version
gcc (Debian 11.3.0-3) 11.3.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.