Please provide preprocessed testcase and gcc options used to compile it.

That said, it can be reproduced even on:
int
foo (int a, int b)
{
  asm volatile ("nop");
  a = 1;
  return a + b;
}
with -g -O2 -mfentry -p -dA.  The issue is that FUNCTION_PROFILER before epilogue is emitted really very early, before processing any NOTE_INSN_VAR_LOCATION that are before the first real insn.
If the targetm.asm_out.function_prologue hook is the default, outputting nothing, perhaps we could consider as ugly hack to process the NOTE_INSN_VAR_LOCATION notes before any non-note insn before calling the FUNCTION_PROFILER, or, just forcefully use the function start label in dwarf2out_var_location for notes that are before first real insn, perhaps limit that to #ifdef HAVE_prologue if (HAVE_prologue) #endif to avoid damaging debug info for !HAVE_prologue targets (if there are still any).

A little longer test case (which requires a location_list rather than location for all the parameters) shows that gdb is adversely affected by this bug too; its prologue-finding is no help:

% cat foo2.c 

int foo (int a, int b) __attribute__((noinline));
int bar (int c, int d) __attribute__((noinline));

int bar (int c, int d)
{
  while (c-- > d++) ;
}

int foo (int a, int b)
{
  asm volatile ("nop");
  return bar (a, b);
}

int main () {
  return foo (7, 4);
}

% gcc -g -O2 -mfentry foo2.c -p
% gdb a.out
(gdb) break foo
Breakpoint 1 at 0x400660: file foo2.c, line 11.
(gdb) run
Starting program: /tmp/a.out 

Breakpoint 1, foo (a=<optimized out>, b=<optimized out>) at foo2.c:11
11	{

I guess the most elegant solution is to have the RTL prologue start with a NOTE_INSN_PROLOGUE_START, and output the before-prologue profiler-call during the final scan of that note, just like we output the after-prologue profiler call at the final scan of NOTE_INSN_PROLOGUE_END.

Well, we can also just use the first NOTE_INSN_BASIC_BLOCK resp. NOTE_INSN_FUNCTION_BEG, whichever comes first.
The bigger problem with that ia64, pa and a few other targets use both
PROLOGUE_BEFORE_EPILOGUE and non-default function_prologue hook, which emits some assembly stuff (even when they HAVE_prologue).  Perhaps for those targets
we'd need to emit the profiler code right away, and only postpone it until the first NOTE_INSN_BASIC_BLOCK/NOTE_INSN_FUNCTION_BEG if function_prologue hook is the default.

Created attachment 29271 [details]
gcc48-pr54793.patch

So, e.g. this seems to work for me from quick testing.  The insn chain scanning is there e.g. to emit the profiler stuff right away when final_start_function is called from output_mi_thunk on various targets.

Author: jakub
Date: Fri Feb  1 16:54:14 2013
New Revision: 195669

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=195669
Log:
	PR debug/54793
	* final.c (need_profile_function): New variable.
	(final_start_function): Drop ATTRIBUTE_UNUSED from first argument.
	If first of NOTE_INSN_BASIC_BLOCK or NOTE_INSN_FUNCTION_BEG
	is only preceeded by NOTE_INSN_VAR_LOCATION or NOTE_INSN_DELETED
	notes, targetm.asm_out.function_prologue doesn't emit anything,
	HAVE_prologue and profiler should be emitted before prologue,
	set need_profile_function instead of emitting it.
	(final_scan_insn): If need_profile_function, emit
	profile_function on the first NOTE_INSN_BASIC_BLOCK or
	NOTE_INSN_FUNCTION_BEG note.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/final.c

Fixed for 4.8+.