[Patch,AVR] Print no-return functions as JMP

[Patch,AVR] Print no-return functions as JMP

[Georg-Johann Lay]

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This patch saves some ticks and bytes on stack by JUMPing to no-return
functions instead of CALLing them.

Passes without regression.

Ok for trunk?

Johann

	* config/avr/avr-protos.h (avr_out_call): New prototype.
	* config/avr/avr.md (adjust_len): Add alternative "call".
	(call_insn, call_calue_insn): Use it.  Use avr_out_call to print
	assembler.
	* config/avr/avr.c (avr_out_call): New function.
	(adjust_insn_length): Handle ADJUST_LEN_CALL.

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Index: config/avr/avr.md
===================================================================
--- config/avr/avr.md	(revision 179843)
+++ config/avr/avr.md	(working copy)
@@ -133,11 +133,10 @@ (define_attr "length" ""
 ;; Following insn attribute tells if and how the adjustment has to be
 ;; done:
 ;;     no     No adjustment needed; attribute "length" is fine.
-;;     yes    Analyse pattern in adjust_insn_length by hand.
 ;; Otherwise do special processing depending on the attribute.
 
 (define_attr "adjust_len"
-  "out_bitop, out_plus, addto_sp, tsthi, tstsi, compare,
+  "out_bitop, out_plus, addto_sp, tsthi, tstsi, compare, call,
    mov8, mov16, mov32, reload_in16, reload_in32,
    ashlqi, ashrqi, lshrqi,
    ashlhi, ashrhi, lshrhi,
@@ -3634,21 +3633,12 @@ (define_insn "call_insn"
   ;; Operand 1 not used on the AVR.
   ;; Operand 2 is 1 for tail-call, 0 otherwise.
   ""
-  "@
-    %!icall
-    %~call %x0
-    %!ijmp
-    %~jmp %x0"
+  {
+     return avr_out_call (insn, operands[0], 0 != INTVAL (operands[2]));
+  }
   [(set_attr "cc" "clobber")
-   (set_attr_alternative "length"
-                         [(const_int 1)
-                          (if_then_else (eq_attr "mcu_mega" "yes")
-                                        (const_int 2)
-                                        (const_int 1))
-                          (const_int 1)
-                          (if_then_else (eq_attr "mcu_mega" "yes")
-                                        (const_int 2)
-                                        (const_int 1))])])
+   (set_attr "length" "1,*,1,*")
+   (set_attr "adjust_len" "*,call,*,call")])
 
 (define_insn "call_value_insn"
   [(parallel[(set (match_operand 0 "register_operand"                   "=r,r,r,r")
@@ -3658,21 +3648,12 @@ (define_insn "call_value_insn"
   ;; Operand 2 not used on the AVR.
   ;; Operand 3 is 1 for tail-call, 0 otherwise.
   ""
-  "@
-    %!icall
-    %~call %x1
-    %!ijmp
-    %~jmp %x1"
+  {
+     return avr_out_call (insn, operands[1], 0 != INTVAL (operands[3]));
+  }
   [(set_attr "cc" "clobber")
-   (set_attr_alternative "length"
-                         [(const_int 1)
-                          (if_then_else (eq_attr "mcu_mega" "yes")
-                                        (const_int 2)
-                                        (const_int 1))
-                          (const_int 1)
-                          (if_then_else (eq_attr "mcu_mega" "yes")
-                                        (const_int 2)
-                                        (const_int 1))])])
+   (set_attr "length" "1,*,1,*")
+   (set_attr "adjust_len" "*,call,*,call")])
 
 (define_insn "nop"
   [(const_int 0)]
Index: config/avr/avr-protos.h
===================================================================
--- config/avr/avr-protos.h	(revision 179842)
+++ config/avr/avr-protos.h	(working copy)
@@ -84,6 +84,7 @@ extern const char *avr_out_sbxx_branch (
 extern const char* avr_out_bitop (rtx, rtx*, int*);
 extern const char* avr_out_plus (rtx*, int*, int*);
 extern const char* avr_out_addto_sp (rtx*, int*);
+extern const char* avr_out_call (rtx, rtx, bool);
 extern bool avr_popcount_each_byte (rtx, int, int);
 
 extern int extra_constraint_Q (rtx x);
Index: config/avr/avr.c
===================================================================
--- config/avr/avr.c	(revision 179843)
+++ config/avr/avr.c	(working copy)
@@ -4905,6 +4905,27 @@ avr_out_plus (rtx *xop, int *plen, int *
 }
 
 
+/* Print call insn INSN to the assembler file and return "".
+   ADDRESS is the target address.
+   If SIBCALL_P then INSN is a tail-call.  */
+   
+const char*
+avr_out_call (rtx insn, rtx address, bool sibcall_p)
+{
+  /* No need to waste stack or time for no-return calls.  */
+  
+  if (optimize && find_reg_note (insn, REG_NORETURN, NULL))
+    sibcall_p = true;
+
+  if (REG_P (address))
+    output_asm_insn (sibcall_p ? "%!ijmp" : "%!icall", &address);
+  else
+    output_asm_insn (sibcall_p ? "%~jmp %x0" : "%~call %x0", &address);
+
+  return "";
+}
+
+
 /* Output bit operation (IOR, AND, XOR) with register XOP[0] and compile
    time constant XOP[2]:
 
@@ -5311,6 +5332,8 @@ adjust_insn_length (rtx insn, int len)
     case ADJUST_LEN_ASHLQI: ashlqi3_out (insn, op, &len); break;
     case ADJUST_LEN_ASHLHI: ashlhi3_out (insn, op, &len); break;
     case ADJUST_LEN_ASHLSI: ashlsi3_out (insn, op, &len); break;
+
+    case ADJUST_LEN_CALL: len = AVR_HAVE_JMP_CALL ? 2 : 1; break;
       
     default:
       gcc_unreachable();

Re: [Patch,AVR] Print no-return functions as JMP

[Richard Henderson]

On 10/13/2011 11:16 AM, Georg-Johann Lay wrote:
> This patch saves some ticks and bytes on stack by JUMPing to no-return
> functions instead of CALLing them.
> 
> Passes without regression.
> 
> Ok for trunk?
> 
> Johann
> 
> 	* config/avr/avr-protos.h (avr_out_call): New prototype.
> 	* config/avr/avr.md (adjust_len): Add alternative "call".
> 	(call_insn, call_calue_insn): Use it.  Use avr_out_call to print
> 	assembler.
> 	* config/avr/avr.c (avr_out_call): New function.
> 	(adjust_insn_length): Handle ADJUST_LEN_CALL.

You should have a way to turn this off.  Otherwise this makes debugging
the call to abort impossible.


r~

Re: [Patch,AVR] Print no-return functions as JMP

[Georg-Johann Lay]

Richard Henderson schrieb:
> On 10/13/2011 11:16 AM, Georg-Johann Lay wrote:
>> This patch saves some ticks and bytes on stack by JUMPing to no-return
>> functions instead of CALLing them.
>>
>> Passes without regression.
>>
>> Ok for trunk?
>>
>> Johann
>>
>> 	* config/avr/avr-protos.h (avr_out_call): New prototype.
>> 	* config/avr/avr.md (adjust_len): Add alternative "call".
>> 	(call_insn, call_calue_insn): Use it.  Use avr_out_call to print
>> 	assembler.
>> 	* config/avr/avr.c (avr_out_call): New function.
>> 	(adjust_insn_length): Handle ADJUST_LEN_CALL.
> 
> You should have a way to turn this off.  Otherwise this makes debugging
> the call to abort impossible.
> 
> r~

What do you propose?

o A command line option that is on per default like
  -mnoreturn-tail-calls or -mjmp-noreturn

o Hard-coded factor out some function names like "abort",
  "exit", "_exit"

Johann

Re: [Patch,AVR] Print no-return functions as JMP

[Richard Henderson]

On 10/13/2011 12:00 PM, Georg-Johann Lay wrote:
> What do you propose?
> 
> o A command line option that is on per default like
>   -mnoreturn-tail-calls or -mjmp-noreturn

The command-line-option.  I think I prefer -mjump-noreturn,
as the inverse -mno-noreturn-tail-calls is too awkward.


r~

RE: [Patch,AVR] Print no-return functions as JMP

[Paul_Koning]

>> You should have a way to turn this off.  Otherwise this makes 
>> debugging the call to abort impossible.
>
>What do you propose?
>
>o A command line option that is on per default like
>  -mnoreturn-tail-calls or -mjmp-noreturn
>
>o Hard-coded factor out some function names like "abort",
>  "exit", "_exit"

I'd suggest the first option.  That way you can do this for other similar functions like panic().

	paul

Re: [Patch,AVR] Print no-return functions as JMP

[Georg-Johann Lay]

Richard Henderson schrieb:
> On 10/13/2011 12:00 PM, Georg-Johann Lay wrote:
> 
>>What do you propose?
>>
>>o A command line option that is on per default like
>>  -mnoreturn-tail-calls or -mjmp-noreturn
> 
> The command-line-option.  I think I prefer -mjump-noreturn,
> as the inverse -mno-noreturn-tail-calls is too awkward.

What about flag_optimize_sibling_calls?
What wa are seeing here is actually a tail call.

Johann

> 
> r~
> 

Re: [Patch,AVR] Print no-return functions as JMP

[Richard Henderson]

On 10/13/2011 11:31 PM, Georg-Johann Lay wrote:
> Richard Henderson schrieb:
>> On 10/13/2011 12:00 PM, Georg-Johann Lay wrote:
>>
>>> What do you propose?
>>>
>>> o A command line option that is on per default like
>>>  -mnoreturn-tail-calls or -mjmp-noreturn
>>
>> The command-line-option.  I think I prefer -mjump-noreturn,
>> as the inverse -mno-noreturn-tail-calls is too awkward.
> 
> What about flag_optimize_sibling_calls?
> What wa are seeing here is actually a tail call.

Because we explicitly don't tail call noreturn for the
reason previously explainted.


r~

Re: [Patch,AVR] Print no-return functions as JMP

[Georg-Johann Lay]

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Richard Henderson schrieb:
> On 10/13/2011 11:31 PM, Georg-Johann Lay wrote:
>> Richard Henderson schrieb:
>>> On 10/13/2011 12:00 PM, Georg-Johann Lay wrote:
>>>
>>>> What do you propose?
>>>>
>>>> o A command line option that is on per default like
>>>>  -mnoreturn-tail-calls or -mjmp-noreturn
>>> The command-line-option.  I think I prefer -mjump-noreturn,
>>> as the inverse -mno-noreturn-tail-calls is too awkward.
>> What about flag_optimize_sibling_calls?
>> What wa are seeing here is actually a tail call.
> 
> Because we explicitly don't tail call noreturn for the
> reason previously explained.

Thanks for the explanation.

Here is the new patch for review with the new option -mjump-to-noreturn

Ok for trunk?

Johann

	* doc/invoke.texi (AVR Options): Document -mjump-to-noreturn.
	
	* config/avr/avr-protos.h (avr_out_call): New prototype.
	* config/avr/avr.md (adjust_len): Add alternative "call".
	(call_insn, call_calue_insn): Use it.  Use avr_out_call to print
	assembler.
	* config/avr/avr.c (avr_out_call): New function.
	(adjust_insn_length): Handle ADJUST_LEN_CALL.

	* common/config/avr/avr-common.c (-mjump-to-noreturn): Turn on for
	-O and higher.


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Index: doc/invoke.texi
===================================================================
--- doc/invoke.texi	(revision 179993)
+++ doc/invoke.texi	(working copy)
@@ -487,7 +487,7 @@ Objective-C and Objective-C++ Dialects}.
 
 @emph{AVR Options}
 @gccoptlist{-mmcu=@var{mcu}  -mno-interrupts @gol
--mcall-prologues  -mtiny-stack  -mint8  -mstrict-X}
+-mcall-prologues  -mtiny-stack  -mint8  -mstrict-X -mjump-to-noreturn}
 
 @emph{Blackfin Options}
 @gccoptlist{-mcpu=@var{cpu}@r{[}-@var{sirevision}@r{]} @gol
@@ -10690,6 +10690,13 @@ and long long will be 4 bytes.  Please n
 comply to the C standards, but it will provide you with smaller code
 size.
 
+@item -mjump-to-noreturn
+@opindex mjump-to-noreturn
+Use a jump instruction instead of a call instruction when calling a
+no-return functions.  This option is active if optimization is turned
+on and just affects the way a call instruction is printed out.
+Besides that, it has no effect on code generation or debug information.
+
 @item -mstrict-X
 @opindex mstrict-X
 Use register @code{X} in a way proposed by the hardware.  This means
Index: config/avr/avr.md
===================================================================
--- config/avr/avr.md	(revision 179992)
+++ config/avr/avr.md	(working copy)
@@ -133,11 +133,10 @@ (define_attr "length" ""
 ;; Following insn attribute tells if and how the adjustment has to be
 ;; done:
 ;;     no     No adjustment needed; attribute "length" is fine.
-;;     yes    Analyse pattern in adjust_insn_length by hand.
 ;; Otherwise do special processing depending on the attribute.
 
 (define_attr "adjust_len"
-  "out_bitop, out_plus, addto_sp, tsthi, tstsi, compare,
+  "out_bitop, out_plus, addto_sp, tsthi, tstsi, compare, call,
    mov8, mov16, mov32, reload_in16, reload_in32,
    ashlqi, ashrqi, lshrqi,
    ashlhi, ashrhi, lshrhi,
@@ -3634,21 +3633,12 @@ (define_insn "call_insn"
   ;; Operand 1 not used on the AVR.
   ;; Operand 2 is 1 for tail-call, 0 otherwise.
   ""
-  "@
-    %!icall
-    %~call %x0
-    %!ijmp
-    %~jmp %x0"
+  {
+    return avr_out_call (insn, operands[0], 0 != INTVAL (operands[2]));
+  }
   [(set_attr "cc" "clobber")
-   (set_attr_alternative "length"
-                         [(const_int 1)
-                          (if_then_else (eq_attr "mcu_mega" "yes")
-                                        (const_int 2)
-                                        (const_int 1))
-                          (const_int 1)
-                          (if_then_else (eq_attr "mcu_mega" "yes")
-                                        (const_int 2)
-                                        (const_int 1))])])
+   (set_attr "length" "1,*,1,*")
+   (set_attr "adjust_len" "*,call,*,call")])
 
 (define_insn "call_value_insn"
   [(parallel[(set (match_operand 0 "register_operand"                   "=r,r,r,r")
@@ -3658,21 +3648,12 @@ (define_insn "call_value_insn"
   ;; Operand 2 not used on the AVR.
   ;; Operand 3 is 1 for tail-call, 0 otherwise.
   ""
-  "@
-    %!icall
-    %~call %x1
-    %!ijmp
-    %~jmp %x1"
+  {
+    return avr_out_call (insn, operands[1], 0 != INTVAL (operands[3]));
+  }
   [(set_attr "cc" "clobber")
-   (set_attr_alternative "length"
-                         [(const_int 1)
-                          (if_then_else (eq_attr "mcu_mega" "yes")
-                                        (const_int 2)
-                                        (const_int 1))
-                          (const_int 1)
-                          (if_then_else (eq_attr "mcu_mega" "yes")
-                                        (const_int 2)
-                                        (const_int 1))])])
+   (set_attr "length" "1,*,1,*")
+   (set_attr "adjust_len" "*,call,*,call")])
 
 (define_insn "nop"
   [(const_int 0)]
Index: config/avr/avr.opt
===================================================================
--- config/avr/avr.opt	(revision 179993)
+++ config/avr/avr.opt	(working copy)
@@ -65,3 +65,7 @@ Make the linker relaxation machine assum
 mstrict-X
 Target Report Var(avr_strict_X) Init(0)
 When accessing RAM, use X as imposed by the hardware, i.e. just use pre-decrement, post-increment and indirect addressing with the X register.  Without this option, the compiler may assume that there is an addressing mode X+const similar to Y+const and Z+const and emit instructions to emulate such an addressing mode for X.
+
+mjump-to-noreturn
+Target Report Var(avr_jump_to_noreturn) Init(0)
+Jump to no-return functions instead of calling them.
Index: config/avr/avr-protos.h
===================================================================
--- config/avr/avr-protos.h	(revision 179992)
+++ config/avr/avr-protos.h	(working copy)
@@ -84,6 +84,7 @@ extern const char *avr_out_sbxx_branch (
 extern const char* avr_out_bitop (rtx, rtx*, int*);
 extern const char* avr_out_plus (rtx*, int*, int*);
 extern const char* avr_out_addto_sp (rtx*, int*);
+extern const char* avr_out_call (rtx, rtx, bool);
 extern bool avr_popcount_each_byte (rtx, int, int);
 
 extern int extra_constraint_Q (rtx x);
Index: config/avr/avr.c
===================================================================
--- config/avr/avr.c	(revision 179993)
+++ config/avr/avr.c	(working copy)
@@ -4928,6 +4928,28 @@ avr_out_plus (rtx *xop, int *plen, int *
 }
 
 
+/* Print call insn INSN to the assembler file and return "".
+   ADDRESS is the target address.
+   If SIBCALL_P then INSN is a tail-call.  */
+   
+const char*
+avr_out_call (rtx insn, rtx address, bool sibcall_p)
+{
+  /* No need to waste stack or time for no-return calls.  */
+  
+  if (avr_jump_to_noreturn
+      && find_reg_note (insn, REG_NORETURN, NULL))
+    sibcall_p = true;
+
+  if (REG_P (address))
+    output_asm_insn (sibcall_p ? "%!ijmp" : "%!icall", &address);
+  else
+    output_asm_insn (sibcall_p ? "%~jmp %x0" : "%~call %x0", &address);
+
+  return "";
+}
+
+
 /* Output bit operation (IOR, AND, XOR) with register XOP[0] and compile
    time constant XOP[2]:
 
@@ -5334,6 +5356,8 @@ adjust_insn_length (rtx insn, int len)
     case ADJUST_LEN_ASHLQI: ashlqi3_out (insn, op, &len); break;
     case ADJUST_LEN_ASHLHI: ashlhi3_out (insn, op, &len); break;
     case ADJUST_LEN_ASHLSI: ashlsi3_out (insn, op, &len); break;
+
+    case ADJUST_LEN_CALL: len = AVR_HAVE_JMP_CALL ? 2 : 1; break;
       
     default:
       gcc_unreachable();
Index: common/config/avr/avr-common.c
===================================================================
--- common/config/avr/avr-common.c	(revision 179765)
+++ common/config/avr/avr-common.c	(working copy)
@@ -29,6 +29,7 @@
 static const struct default_options avr_option_optimization_table[] =
   {
     { OPT_LEVELS_1_PLUS, OPT_fomit_frame_pointer, NULL, 1 },
+    { OPT_LEVELS_1_PLUS, OPT_mjump_to_noreturn, NULL, 1 },
     { OPT_LEVELS_NONE, 0, NULL, 0 }
   };
 

Re: [Patch,AVR] Print no-return functions as JMP

[Paolo Bonzini]

On 10/14/2011 06:23 PM, Georg-Johann Lay wrote:
> +@item -mjump-to-noreturn
> +@opindex mjump-to-noreturn
> +Use a jump instruction instead of a call instruction when calling a
> +no-return functions.  This option is active if optimization is turned
> +on and just affects the way a call instruction is printed out.
> +Besides that, it has no effect on code generation or debug information.

I think this is not really accurate given Richard's input.

Paolo

Re: [Patch,AVR] Print no-return functions as JMP

[Georg-Johann Lay]

Paolo Bonzini schrieb:
> On 10/14/2011 06:23 PM, Georg-Johann Lay wrote:
>> +@item -mjump-to-noreturn
>> +@opindex mjump-to-noreturn
>> +Use a jump instruction instead of a call instruction when calling a
>> +no-return functions.  This option is active if optimization is turned
>> +on and just affects the way a call instruction is printed out.
>> +Besides that, it has no effect on code generation or debug information.
> 
> I think this is not really accurate given Richard's input.
> 
> Paolo
> 

Confused.  The conclusion was to introduce a new command line option in order
to have individual control over this feature.  The option is named
-mjump-to-noreturn now instead of -mjump-noreturn. Is that what you mean?

Johann

Re: [Patch,AVR] Print no-return functions as JMP

[Paolo Bonzini]

On Fri, Oct 14, 2011 at 19:19, Georg-Johann Lay <avr@gjlay.de> wrote:
> Paolo Bonzini schrieb:
>> On 10/14/2011 06:23 PM, Georg-Johann Lay wrote:
>>> +@item -mjump-to-noreturn
>>> +@opindex mjump-to-noreturn
>>> +Use a jump instruction instead of a call instruction when calling a
>>> +no-return functions.  This option is active if optimization is turned
>>> +on and just affects the way a call instruction is printed out.
>>> +Besides that, it has no effect on code generation or debug information.
>>
>> I think this is not really accurate given Richard's input.
>
> Confused.  The conclusion was to introduce a new command line option in order
> to have individual control over this feature.  The option is named
> -mjump-to-noreturn now instead of -mjump-noreturn. Is that what you mean?

No, I mean that it has an effect on debuggability.  You will not be
able to derive the place where you crashed from the backtrace.

Paolo

Re: [Patch,AVR] Print no-return functions as JMP

[Pedro Alves]

On Friday 14 October 2011 18:19:00, Georg-Johann Lay wrote:
> Paolo Bonzini schrieb:
> > On 10/14/2011 06:23 PM, Georg-Johann Lay wrote:
> >> +@item -mjump-to-noreturn
> >> +@opindex mjump-to-noreturn
> >> +Use a jump instruction instead of a call instruction when calling a
> >> +no-return functions.  This option is active if optimization is turned
> >> +on and just affects the way a call instruction is printed out.
> >> +Besides that, it has no effect on code generation or debug information.
> > 
> > I think this is not really accurate given Richard's input.
> 
> Confused.  The conclusion was to introduce a new command line option in order
> to have individual control over this feature.  The option is named
> -mjump-to-noreturn now instead of -mjump-noreturn. Is that what you mean?

My 2c.  You've used implementor-speak to describe the option, while you
should use user-speak.  Mention that it saves stack, but the downside is
that it affects backtracing (and suggest turning it off if you want to
backtrace out of abort, etc.).  Lose the "just affects the way a call
instruction is printed out" bit -- what's "printed out"?, a user
would ask.  Some users aren't even aware there's a separate assembler
that munches text is involved.

I'd also suggest renaming the option to `-mallow-tailcall-noreturn'
or `-mtailcall-noreturn' so that its spelling and description could
be more easily shared with other targets (and perhaps promoted
as -f option).

-- 
Pedro Alves

Re: [Patch,AVR] Print no-return functions as JMP

[Gerald Pfeifer]

 
+@item -mjump-to-noreturn
+@opindex mjump-to-noreturn
+Use a jump instruction instead of a call instruction when calling a
+no-return functions.  This option is active if optimization is turned
+on and just affects the way a call instruction is printed out.

Would "emit" be better here than "printed out"?

Gerald

Re: [Patch,AVR] Print no-return functions as JMP

[Richard Kenner]

> +@item -mjump-to-noreturn
> +@opindex mjump-to-noreturn
> +Use a jump instruction instead of a call instruction when calling a
> +no-return functions.  This option is active if optimization is turned
> +on and just affects the way a call instruction is printed out.
> 
> Would "emit" be better here than "printed out"?

Maybe "generated"?

Re: [Patch,AVR] Print no-return functions as JMP

[Georg-Johann Lay]

Pedro Alves schrieb:
> On Friday 14 October 2011 18:19:00, Georg-Johann Lay wrote:
> 
>>Paolo Bonzini schrieb:
>>
>>>On 10/14/2011 06:23 PM, Georg-Johann Lay wrote:
>>>
>>>>+@item -mjump-to-noreturn
>>>>+@opindex mjump-to-noreturn
>>>>+Use a jump instruction instead of a call instruction when calling a
>>>>+no-return functions.  This option is active if optimization is turned
>>>>+on and just affects the way a call instruction is printed out.
>>>>+Besides that, it has no effect on code generation or debug information.
>>>
>>>I think this is not really accurate given Richard's input.
>>
>>Confused.  The conclusion was to introduce a new command line option in order
>>to have individual control over this feature.  The option is named
>>-mjump-to-noreturn now instead of -mjump-noreturn. Is that what you mean?
> 
> My 2c.  You've used implementor-speak to describe the option, while you
> should use user-speak.  Mention that it saves stack, but the downside is
> that it affects backtracing (and suggest turning it off if you want to
> backtrace out of abort, etc.).  Lose the "just affects the way a call
> instruction is printed out" bit -- what's "printed out"?, a user
> would ask.  Some users aren't even aware there's a separate assembler
> that munches text is involved.

Thanks for your input. I am always grateful for hints on how to improve 
my english.

> I'd also suggest renaming the option to `-mallow-tailcall-noreturn'
> or `-mtailcall-noreturn' so that its spelling and description could
> be more easily shared with other targets (and perhaps promoted
> as -f option).

Looking at -foptimize-sibling-calls there is just

-foptimize-sibling-calls
	Optimize sibling and tail recursive calls.

	Enabled at levels -O2, -O3, -Os.

so the new option could be

-moptimize-noreturn-calls
	Optimize noreturn calls.  This might make debugging harder but
	will save storing the return address when calling roreturn
	functions.

	Enabled at levels -O2, -O3, -Os.

But the "makes debugging harder" clause is true for almost any 
optimization available, so should this tautology be mentioned along with 
each optimization that makes debugging harder?

Or simply like this:

-moptimize-noreturn-calls
	Optimize noreturn calls.

	Enabled at levels -O2, -O3, -Os.

What about the -moptimize-noreturn-calls in analogy to 
-foptimize-sibling-calls? Sound good to me.

Johann

Re: [Patch,AVR] Print no-return functions as JMP

[Paolo Bonzini]

> -moptimize-noreturn-calls
>        Optimize noreturn calls.  This might make debugging harder but
>        will save storing the return address when calling roreturn
>        functions.
>
>        Enabled at levels -O2, -O3, -Os.
>
> But the "makes debugging harder" clause is true for almost any optimization
> available, so should this tautology be mentioned along with each
> optimization that makes debugging harder?

If there's one kind of debugging that you don't want to make harder,
that's post mortem debugging.

Paolo

Re: [Patch,AVR] Print no-return functions as JMP

[Georg-Johann Lay]

Paolo Bonzini schrieb:
>>-moptimize-noreturn-calls
>>       Optimize noreturn calls.  This might make debugging harder but
>>       will save storing the return address when calling roreturn
>>       functions.
>>
>>       Enabled at levels -O2, -O3, -Os.
>>
>>But the "makes debugging harder" clause is true for almost any optimization
>>available, so should this tautology be mentioned along with each
>>optimization that makes debugging harder?
> 
> If there's one kind of debugging that you don't want to make harder,
> that's post mortem debugging.
> 
> Paolo

And what is the conclusion?

- Not to imlement this optimization at all?
- Implement it but don't turn it on depending on -O*
- Implement it as is but explicitely mention
   "post mortem debugging" in docs?

There is no real post morten debugging on AVR as there is nothing like 
core dump.

Johann

RE: [Patch,AVR] Print no-return functions as JMP

[Paul_Koning]

>There is no real post morten debugging on AVR as there is nothing like core dump.

But if there is any kind of debugging at all, you might use the debugger to put a breakpoint in abort(), and if so, having that invoked via jmp means you don't know who called it.  So you'd want a way to suppress that optimization.

	paul

Re: [Patch,AVR] Print no-return functions as JMP

[Georg-Johann Lay]

Paul_Koning@Dell.com schrieb:
>> There is no real post morten debugging on AVR as there is nothing like
>> core dump.
> 
> But if there is any kind of debugging at all, you might use the debugger to
> put a breakpoint in abort(), and if so, having that invoked via jmp means
> you don't know who called it.  So you'd want a way to suppress that
> optimization.
> 
> paul

Regarding the number of objections, I think it's best to drop this patch.

The clean-up to the call insns is contained in
http://gcc.gnu.org/ml/gcc-patches/2011-10/msg01530.html

Thanks for all of your feedback!

Johann