Compiler Warning in case of -O3 or -O2

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ce
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Posted by ce: Mon. Oct 29, 2007 - 04:03 PM
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I like to get rid of a compiler warning which only appears with the optimization switch -O2 or -O3.

This is the warning:
protocol.c:111: warning: dereferencing type-punned pointer will break strict-aliasing rules

This is the prototype of a function which call causes the warning:

extern unsigned char get_cobs_tx_buf(unsigned char **tx_buf);

For this function call I get the above mentioned warning:

t2m_error_t *msg;
  unsigned char idx;

  idx=get_cobs_tx_buf((unsigned char **)(&msg));

But the following function call does not cause a warning:

unsigned char *msg;
  unsigned char idx;

  idx=get_cobs_tx_buf(&msg);

1. Am I doing anything wrong? (Although the code seems to work...)
2. Is it possible to change the optimization level by a #pragma?
3. Can I supress a certain warning or a group of warnings by a #pragma?

Thanks in advance!

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Tools, Compilers and General Programming
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Bronze MarkJohanEkdahl
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Posted by JohanEkdahl: Mon. Oct 29, 2007 - 04:10 PM
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How is t2m_error_t defined?

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ce
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Posted by ce: Mon. Oct 29, 2007 - 04:29 PM
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Tjena!

typedef struct
{
  unsigned char rec_type;
  unsigned char status;
}t2m_error_t;
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Bronze MarkJohanEkdahl
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Posted by JohanEkdahl: Mon. Oct 29, 2007 - 05:44 PM
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Hejdu! (Excuse us for our subversive act of greeting in swedish)

1: Do a search for "dereferencing type-punned pointer will break" at Google. It will give you heaps of explanations of the warning message, eg. http://mail.opensolaris.org/pipe... . Essentially it is a warning that different types aren't stored in the same way, and in extension this means that a pointer of type

t2m_error_t * *

is not (guaranteed to be) storage compatible with

unsigned char * *

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ce
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Posted by ce: Tue. Oct 30, 2007 - 10:06 AM
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Thanks Johan (not to say 'tack'),

I chose the pragmatic way and use the compiler option -fno-strict-aliasing. My code size (.text) increased from 14288 to 14304 bytes. This is acceptable.

But have a look at point 2 and 3 of my introducing post. Is there a way to control the compiler behaviour by pragmas? I haven't found anything for GCC. I know that other compilers have this ability.

Regards,
ce

OT: I was a couple of months in Sweden. But I only learned to say 'hello', 'two beers, please' and 'thanks'. ;-)

Last Edited: Tue. Oct 30, 2007 - 10:26 AM
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Gold Markclawson
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Posted by clawson: Tue. Oct 30, 2007 - 10:21 AM
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No GCC does not have (many) #pragmas

http://gcc.gnu.org/onlinedocs/gc...

So, to answer (2), if you want to change the optimisation level of a certain function then put it in a separate source file and change the compile command for that file to use a different optimisation setting (course you cannot isolate the change of optimisation to just a few lines using this method, only an entire function)

As for (3), you have the following options for changing specific warning behaviour:

http://gcc.gnu.org/onlinedocs/gc...

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ce
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Posted by ce: Tue. Oct 30, 2007 - 10:30 AM
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Thanks,

this is what I also found. But I searched for pragmas supressing warnings or changing compiler optimization levels. But it's hard to find something which does not exist. :-)

Thanks to all of you!

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EW
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Posted by EW: Wed. Oct 31, 2007 - 03:27 AM
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Note: GCC does not *yet* have the ability to change optimization levels per function. However, I know that it is on a GCC developer's todo list to implement that feature at some point. I've talked to him and mentioned that that feature would be highly desirable in the embedded community. But, like all open source projects, it's a matter of when volunteers get around to implementing the feature.

Eric Weddington
www.linkedin.com/in/ericweddington/

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abcminiuser
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Posted by abcminiuser: Wed. Oct 31, 2007 - 03:47 AM
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Quote:
Note: GCC does not *yet* have the ability to change optimization levels per function. However, I know that it is on a GCC developer's todo list to implement that feature at some point. I've talked to him and mentioned that that feature would be highly desirable in the embedded community. But, like all open source projects, it's a matter of when volunteers get around to implementing the feature.

I remember seeing the new "hot" and "cold" attributes for GCC4.3, which allow for per-function optimization. Was I seeing things?

- Dean :twisted:

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EW
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Posted by EW: Wed. Oct 31, 2007 - 04:51 PM
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abcminiuser wrote:

I remember seeing the new "hot" and "cold" attributes for GCC4.3, which allow for per-function optimization. Was I seeing things?

Yeah, don't even go there. I don't think that stuff is valid for the AVR.

Eric Weddington
www.linkedin.com/in/ericweddington/

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