Runtime Parameters in GCC

Runtime Parameters in GCC

[SANKAR]

Hi, 

I am exploring the runtime parameters of GCC from params.def file. 

I have the following queries with regard to this: 

1. It is observed that for many parameters(almost for 90% ) the max value is
given as '0'. 

How to interepret this? What is the real Maximum value that these parameters
can take? 

2. For some parameters like max-sched-extend-regions-iters, 
max-fields-for-field-sensitive etc., the default, min and max value all are
'0' (0,0,0). Then 

what is the necessity to be the parameter, rather it can be the constant. 

3. For parameter 'integer-share-limit' the default value is 251 and the max
and min values are 2,2. Can you explain how to interpret? 

4. The default value for "max-inline-insns-single" is given as 400 in the
DEFPARAM, whereas the in the comment the default value mentioned as 450. 

5. I hope the values that these parameters can take are discrete numbers
only. Then for some parameters where the possible values are binary, for
example  omega-eliminate-redundant-constraints (0,0,1),
use-canonical-types(1,0,1) etc., can they be converted -fno/-f style
options? 


Thanks in advance, 

Sankar



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Re: Runtime Parameters in GCC

[Ian Lance Taylor]

On Mon, Nov 5, 2012 at 7:58 PM, SANKAR <chnabsankar@yahoo.com> wrote:
>
> 1. It is observed that for many parameters(almost for 90% ) the max value is
> given as '0'.
>
> How to interepret this? What is the real Maximum value that these parameters
> can take?

It means that the compiler does not enforce any maximum value when you
use --param.  The meaning of very large values depends on the
parameter.

> 2. For some parameters like max-sched-extend-regions-iters,
> max-fields-for-field-sensitive etc., the default, min and max value all are
> '0' (0,0,0). Then
>
> what is the necessity to be the parameter, rather it can be the constant.

As the documentation says, the real default value for
max-fields-for-field-sensitive depends on the optimization level.  The
parameter has a meaning.  As the comment in params.def says, the
maximum value is only meaningful if it is larger than the minimum
value.  In this case the parameter has a minimum value of zero and no
maximum.

> 3. For parameter 'integer-share-limit' the default value is 251 and the max
> and min values are 2,2. Can you explain how to interpret?

The minimum is 2 and there is no maximum.

> 4. The default value for "max-inline-insns-single" is given as 400 in the
> DEFPARAM, whereas the in the comment the default value mentioned as 450.

Sounds like a bug.  Typically when the code and the comment disagree,
you should go with the code.

> 5. I hope the values that these parameters can take are discrete numbers
> only. Then for some parameters where the possible values are binary, for
> example  omega-eliminate-redundant-constraints (0,0,1),
> use-canonical-types(1,0,1) etc., can they be converted -fno/-f style
> options?

Sure, they could.  Since -f options can take arguments, any --param
option could be implemented as a -f option.  Generally --param is used
for parameters that are only of interest to GCC maintainers.  -f
options are intended for users.

Ian

Re: Runtime Parameters in GCC

[SANKAR]

Thanks Ian.

If the classification of these parameters with respect to their influence on
Optimization Objective(Compilation time/memory, Execution time, Code size)
etc is provided, the user can fine tune their application further. 

Is there any list of Language specific and Architecture specific Runtime
Parameter list?

Thanks
Sankar



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Re: Runtime Parameters in GCC

[Ian Lance Taylor]

On Tue, Nov 6, 2012 at 1:38 AM, SANKAR <chnabsankar@yahoo.com> wrote:
>
> If the classification of these parameters with respect to their influence on
> Optimization Objective(Compilation time/memory, Execution time, Code size)
> etc is provided, the user can fine tune their application further.

All the parameters are documented in the manual.  Many of them are
tradeoffs among the areas you mention.

> Is there any list of Language specific and Architecture specific Runtime
> Parameter list?

As far as I know, the only language-specific parameter is
cxx-max-namespaces-for-diagnostic-help.  As far as I know, there are
no architecture-specific parameters, though it may be worth mentioning
that several of the backends do set default values for parameters like
l1-cache-size.

Ian