Re: [avr] gas support for cfi info

[Richard Henderson <rth@redhat.com>]

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On 02/15/2011 02:44 PM, Petr Hluzín wrote:
> In avr-tdep.c [1] near avr_dwarf_reg_to_regnum():
> /* Unfortunately dwarf2 register for SP is 32.  */

Excellent.  We're all on the same page for this.

> (I can't help you with the value for #define DWARF2_DEFAULT_RETURN_COLUMN  36)
> AFAIK there is no written ABI. Only the calling convention is
> documented (and only the easy cases), the rest is in gdb/gcc/binutils
> sources and people's heads.

As I recall, GCC defaults to using FIRST_PSEUDO_REGISTER for this,
so as to not overlap any hard registers.  I'll continue to so the same.

>   /* Avr-6 call instructions save 3 bytes.  */
>   switch (info.bfd_arch_info->mach)

Thanks.  That value is readily available in the assembler as well.

Anitha pointed out to me via gcc pr17994 that AVR uses post-decrement
for its pushes.  I had a brief read over the AVR insn manual, and it's
not crystal clear how multi-byte post-decrement pushes operate.

I've made an assumption that it happens as-if each byte is pushed
separately.  I.e.

  caller:           callee:
    save rN
    save rM
    trash    <- SP  hi(ret)  <- CFA
                    lo(ret)
                    trash    <- SP

This is the only way I can imagine that call insns interoperate with
byte push/pop insns.

All of which means that the return address is at a different offset
from the CFA than I originally thought.  This ought to be fixed in
the following.

Can someone please test these two patches and see if they actually
agree with the hardware?


r~

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Index: config/tc-avr.c
===================================================================
RCS file: /cvs/src/src/gas/config/tc-avr.c,v
retrieving revision 1.74
diff -u -p -r1.74 tc-avr.c
--- config/tc-avr.c	28 Jun 2010 14:06:57 -0000	1.74
+++ config/tc-avr.c	16 Feb 2011 17:39:55 -0000
@@ -24,6 +24,8 @@
 #include "as.h"
 #include "safe-ctype.h"
 #include "subsegs.h"
+#include "dw2gencfi.h"
+
 
 struct avr_opcodes_s
 {
@@ -1488,3 +1490,18 @@ avr_cons_fix_new (fragS *frag,
       exp_mod_pm = 0;
     }
 }
+
+void
+tc_cfi_frame_initial_instructions (void)
+{
+  /* AVR6 pushes 3 bytes for calls.  */
+  int return_size = (avr_mcu->mach == bfd_mach_avr6 ? 3 : 2);
+
+  /* The CFA is the caller's stack location before the call insn.  */
+  /* Note that the stack pointer is dwarf register number 32.  */
+  cfi_add_CFA_def_cfa (32, return_size);
+
+  /* Note that AVR consistently uses post-decrement, which means that things
+     do not line up the same way as for targers that use pre-decrement.  */
+  cfi_add_CFA_offset (DWARF2_DEFAULT_RETURN_COLUMN, 1-return_size);
+}
Index: config/tc-avr.h
===================================================================
RCS file: /cvs/src/src/gas/config/tc-avr.h,v
retrieving revision 1.17
diff -u -p -r1.17 tc-avr.h
--- config/tc-avr.h	27 Oct 2009 15:39:27 -0000	1.17
+++ config/tc-avr.h	16 Feb 2011 17:39:55 -0000
@@ -153,3 +153,16 @@ extern long md_pcrel_from_section (struc
 
 /* 32 bits pseudo-addresses are used on AVR.  */
 #define DWARF2_ADDR_SIZE(bfd) 4
+
+/* Enable cfi directives.  */
+#define TARGET_USE_CFIPOP 1
+
+/* The stack grows down, and is only byte aligned.  */
+#define DWARF2_CIE_DATA_ALIGNMENT -1
+
+/* Define the column that represents the PC.  */
+#define DWARF2_DEFAULT_RETURN_COLUMN  36
+
+/* Define a hook to setup initial CFI state.  */
+extern void tc_cfi_frame_initial_instructions (void);
+#define tc_cfi_frame_initial_instructions tc_cfi_frame_initial_instructions

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diff --git a/gcc/config/avr/avr-protos.h b/gcc/config/avr/avr-protos.h
index 4569359..06c9254 100644
--- a/gcc/config/avr/avr-protos.h
+++ b/gcc/config/avr/avr-protos.h
@@ -109,6 +109,7 @@ extern RTX_CODE avr_normalize_condition (RTX_CODE condition);
 extern int compare_eq_p (rtx insn);
 extern void out_shift_with_cnt (const char *templ, rtx insn,
 				rtx operands[], int *len, int t_len);
+extern rtx avr_incoming_return_addr_rtx (void);
 #endif /* RTX_CODE */
 
 #ifdef HAVE_MACHINE_MODES
diff --git a/gcc/config/avr/avr.c b/gcc/config/avr/avr.c
index 30e4626..761eea3 100644
--- a/gcc/config/avr/avr.c
+++ b/gcc/config/avr/avr.c
@@ -534,6 +534,16 @@ get_sequence_length (rtx insns)
   return length;
 }
 
+/*  Implement INCOMING_RETURN_ADDR_RTX.  */
+
+rtx
+avr_incoming_return_addr_rtx (void)
+{
+  /* The return address is at the top of the stack.  Note that the push
+     was via post-decrement, which means the actual address is off by one.  */
+  return gen_frame_mem (HImode, plus_constant (stack_pointer_rtx, 1));
+}
+
 /*  Output function prologue.  */
 
 void
diff --git a/gcc/config/avr/avr.h b/gcc/config/avr/avr.h
index 9a71078..385b4e6 100644
--- a/gcc/config/avr/avr.h
+++ b/gcc/config/avr/avr.h
@@ -809,6 +809,9 @@ mmcu=*:-mmcu=%*}"
 
 #define OBJECT_FORMAT_ELF
 
+#define INCOMING_RETURN_ADDR_RTX   avr_incoming_return_addr_rtx ()
+#define INCOMING_FRAME_SP_OFFSET   (AVR_3_BYTE_PC ? 3 : 2)
+
 #define HARD_REGNO_RENAME_OK(OLD_REG, NEW_REG) \
   avr_hard_regno_rename_ok (OLD_REG, NEW_REG)
 
diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index fea8209..b5e660c 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -2240,7 +2240,7 @@ dwarf2out_frame_debug_cfa_restore (rtx reg, const char *label)
 	   cfa.base_offset = -cfa_store.offset
 
   Rule 11:
-  (set (mem ({pre_inc,pre_dec} sp:cfa_store.reg)) <reg>)
+  (set (mem ({pre_inc,pre_dec,post_dec} sp:cfa_store.reg)) <reg>)
   effects: cfa_store.offset += -/+ mode_size(mem)
 	   cfa.offset = cfa_store.offset if cfa.reg == sp
 	   cfa.reg = sp
@@ -2259,7 +2259,7 @@ dwarf2out_frame_debug_cfa_restore (rtx reg, const char *label)
 	   cfa.base_offset = -{cfa_store,cfa_temp}.offset
 
   Rule 14:
-  (set (mem (postinc <reg1>:cfa_temp <const_int>)) <reg2>)
+  (set (mem (post_inc <reg1>:cfa_temp <const_int>)) <reg2>)
   effects: cfa.reg = <reg1>
 	   cfa.base_offset = -cfa_temp.offset
 	   cfa_temp.offset -= mode_size(mem)
@@ -2592,6 +2592,7 @@ dwarf2out_frame_debug_expr (rtx expr, const char *label)
 	  /* Rule 11 */
 	case PRE_INC:
 	case PRE_DEC:
+	case POST_DEC:
 	  offset = GET_MODE_SIZE (GET_MODE (dest));
 	  if (GET_CODE (XEXP (dest, 0)) == PRE_INC)
 	    offset = -offset;
@@ -2616,7 +2617,10 @@ dwarf2out_frame_debug_expr (rtx expr, const char *label)
 	  if (cfa.reg == STACK_POINTER_REGNUM)
 	    cfa.offset = cfa_store.offset;
 
-	  offset = -cfa_store.offset;
+	  if (GET_CODE (XEXP (dest, 0)) == POST_DEC)
+	    offset += -cfa_store.offset;
+	  else
+	    offset = -cfa_store.offset;
 	  break;
 
 	  /* Rule 12 */