[RFA/RFC] ARM : one-line prologue analysis

[Jerome Guitton <guitton@act-europe.fr>]

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The support for unwinding through functions which do not build a frame
pointer seems broken for arm-elf. Consider the attached example. It is
basically main -> c -> b -> a (X '->' Y meaning "X calls Y"). the
calls to printf are here only to avoid sibcall optimization. With gcc
3.2, we will get a one-line prologue for a, b, c and main :


a:
        str     lr, [sp, #-4]!
	[...]

Alas, getting a backtrace from 'a' will not work properly:

(gdb) file d
Reading symbols from d...done.
(gdb) target sim
Connected to the simulator.
(gdb) load d
[...]
(gdb) b a
Breakpoint 1 at 0x8228: file a.c, line 5.
(gdb) r
Starting program: /cardiff.a/guitton/fsf/gdb-head-merge/tmp/test/d 

Breakpoint 1, a () at a.c:5
5         printf ("HERE I AM J.H.");
(gdb) bt
#0  a () at a.c:5
#1  0x00824000 in ?? ()

There is a 'address minus one' thing going on. Indeed, 0x00824000 is not a
valid address, it should be 0x00008240:

(gdb) x/i 0x00008240
0x8240 <b+8>:   ldr     r0, [pc, #8]    ; 0x8250 <b+24>

This problem is in arm_make_prologue_cache. Indeed, for testing if a
register has been saved on the stack, arm_make_prologue_cache should
compare the field addr of the saved reg table to -1, according to
trad-frame.h.  It compare it to 0! That's where the 'address minus -1'
thing comes from.

The first patch is a fix for this bug (obv.dif). It seems straight forward
to me. I run the gdb testsuite on a arm-elf simulator, it fixes 150 failures.
No regression.

Still, there is another bug:

(gdb) bt
#0  a () at a.c:5
#1  0x00008240 in b () at b.c:4
#2  0x00008240 in b () at b.c:4

For some reason that I don't understand completely (and that's the
[RFC] part of this email), in arm_scan_prologue the choice has been
made not to get lr from the stack if the prologue is "str lr, [sp,
#4]!". I just want to point out that getting it from the stack (my
second patch, lr.dif) fixes the problem:

(gdb) bt
#0  a () at a.c:5
#1  0x00008240 in b () at b.c:4
#2  0x0000825c in c () at c.c:4
#3  0x00008278 in main () at d.c:2

I don't thing that's a good idea to consider that these one-line-prologue
functions are frameless, even if they don't build a frame pointer.
Opinions/ideas?

Tested against the testsuite on the arm simulator, I get no regression and
no fix. (the baseline debugger includes my first patch)

-- 
Jerome

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2003-09-25  Jerome Guitton  <guitton@act-europe.fr>

	* arm-tdep.c (arm_make_prologue_cache): Compare the field addr of
	the saved regs record with -1 to test if the register has been saved
	on the stack.

*** arm-tdep.c.1	Tue Sep 23 20:58:12 2003
--- arm-tdep.c	Tue Sep 23 20:58:45 2003
*************** arm_make_prologue_cache (struct frame_in
*** 967,973 ****
    /* Calculate actual addresses of saved registers using offsets
       determined by arm_scan_prologue.  */
    for (reg = 0; reg < NUM_REGS; reg++)
!     if (cache->saved_regs[reg].addr != 0)
        cache->saved_regs[reg].addr += cache->prev_sp;
  
    return cache;
--- 967,973 ----
    /* Calculate actual addresses of saved registers using offsets
       determined by arm_scan_prologue.  */
    for (reg = 0; reg < NUM_REGS; reg++)
!     if (cache->saved_regs[reg].addr != -1)
        cache->saved_regs[reg].addr += cache->prev_sp;
  
    return cache;

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2003-09-25  Jerome Guitton  <guitton@act-europe.fr>

	* arm-tdep.c (arm_scan_prologue_cache): When analysing the instruction
	"str lr, [sp, #-4]", save the address where lr has been stored.

*** arm-tdep.c.2	Tue Sep 23 20:59:52 2003
--- arm-tdep.c	Tue Sep 23 21:06:07 2003
*************** arm_scan_prologue (struct frame_info *ne
*** 845,851 ****
  	}
        else if (insn == 0xe52de004)	/* str lr, [sp, #-4]! */
  	{
! 	  /* Function is frameless: extra_info defaults OK?  */
  	  continue;
  	}
        else if ((insn & 0xffff0000) == 0xe92d0000)
--- 845,852 ----
  	}
        else if (insn == 0xe52de004)	/* str lr, [sp, #-4]! */
  	{
! 	  sp_offset -= 4;
! 	  cache->saved_regs[ARM_LR_REGNUM].addr = sp_offset;
  	  continue;
  	}
        else if ((insn & 0xffff0000) == 0xe92d0000)

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/* a.c */


extern void abort ();

void a () {
  printf ("HERE I AM J.H.");
}

/* b.c */

void b () {
  int i = 2;

  a ();
  printf ("END B %d \n", i);
}

/* c.c */

void c () {
  int i = 3;

  b ();
  printf ("END C %d \n", i);
}

/* d.c */

int main () {
  c ();
  printf ("END D\n");
}

#Makefile

CFLAGS=-O2 -save-temps -g
ARMCFLAGS=-mno-apcs-frame

.SUFFIXES: .c

.c.o:
	arm-elf-gcc -c $(CFLAGS) $(ARMCFLAGS) $<

d: a.o b.o c.o d.o
	arm-elf-gcc a.o b.o c.o d.o -o d

all: d

clean:
	-rm *.o