linux-kernel - Re: [RFC PATCH 3/4] gcov: compile specific gcov implementation based on gcc version

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Message-ID: <521B5064.3000605@linux.vnet.ibm.com>
Date:	Mon, 26 Aug 2013 14:56:04 +0200
From:	Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>
To:	Frantisek Hrbata <fhrbata@...hat.com>
CC:	linux-kernel@...r.kernel.org, jstancek@...hat.com,
	keescook@...omium.org, rusty@...tcorp.com.au,
	linux-arch@...r.kernel.org, arnd@...db.de, mgahagan@...hat.com,
	agospoda@...hat.com
Subject: Re: [RFC PATCH 3/4] gcov: compile specific gcov implementation based
 on gcc version

On 24.08.2013 21:12, Frantisek Hrbata wrote:
> On Fri, Aug 23, 2013 at 05:15:19PM +0200, Peter Oberparleiter wrote:
>> Also it is my understanding that there are some distribution-specific versions
>> of GCC that include the 4.7. gcov format code but report GCC version 4.6. With
>> the auto-detection code implemented like this, gcov-kernel won't work correctly.
>> For that purpose I've implemented a configuration option to allow users to
>> force a specific version of gcov format.
> 
> Ah, I was not aware of this inconsistency in versioning. This raises a question
> if it would not be better to deal directly with version in the gcov_info
> instead of these config options. This would of course mean some kind of gcov
> operations callbacks per gcov version(you already mentioned the file
> operations approach).

Using the version information from gcov_info would make the support easier to use,
but I see 2 problems with this approach:

1. How do you decide which version applies to any given gcov_info struct?
Since gcov_info is opaque this would require a compatibility check inside
the version specific code. And each version specific code assumes a different
layout for gcov_info, so in an (unlikely) worst case scenario, the compatibility
check might fail (false positive or access beyond end of data structure) due to
these differences

2. There's no easy way to "force" compatibility, for example in the case
mentioned above where a GCC 4.6 produces GCC 4.7 gcov format data.

>> I'm attaching the corresponding patch below:
>>
>> ---
>> kernel: gcov: make data format configurable
>>
>> Make the format of the generated gcov data configurable. This may be
>> required for example for pre-4.7 GCCs that contain the 4.7 gcov data
>> format changes.
>>
>> Signed-off-by: Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>
>> ---
>>  kernel/gcov/Kconfig  |   30 ++++++++++++++++++++++++++++++
>>  kernel/gcov/Makefile |   21 +++++++++++++++++++--
>>  2 files changed, 49 insertions(+), 2 deletions(-)
>>
>> --- a/kernel/gcov/Kconfig
>> +++ b/kernel/gcov/Kconfig
>> @@ -46,4 +46,34 @@ config GCOV_PROFILE_ALL
>>  	larger and run slower. Also be sure to exclude files from profiling
>>  	which are not linked to the kernel image to prevent linker errors.
>>
>> +choice
>> +	prompt "Specify GCOV format"
>> +	depends on GCOV_KERNEL
>> +	default GCOV_FORMAT_AUTODETECT
>> +	---help---
>> +	The gcov format is usually determined by the GCC version, but there are
>> +	exceptions where format changes are integrated in lower-version GCCs.
>> +	In such a case use this option to adjust the format used in the kernel
>> +	accordingly.
>> +
>> +	If unsure, choose "Autodetect".
>> +
>> +config GCOV_FORMAT_AUTODETECT
>> +	bool "Autodetect"
>> +	---help---
>> +	Select this option to use the format that corresponds to your GCC
>> +	version.
>> +
>> +config GCOV_FORMAT_3_4
>> +	bool "GCC 3.4 format"
>> +	---help---
>> +	Select this option to use the format defined by GCC 3.4.
>> +
>> +config GCOV_FORMAT_4_7
>> +	bool "GCC 4.7 format"
>> +	---help---
>> +	Select this option to use the format defined by GCC 4.7.
>> +
>> +endchoice
>> +
>>  endmenu
>> --- a/kernel/gcov/Makefile
>> +++ b/kernel/gcov/Makefile
>> @@ -1,5 +1,22 @@
>>  ccflags-y := -DSRCTREE='"$(srctree)"' -DOBJTREE='"$(objtree)"'
>>
>> +# if-lt
>> +# Usage VAR := $(call if-lt, $(a), $(b))
>> +# Returns 1 if (a < b)
>> +if-lt = $(shell [ $(1) -lt $(2) ] && echo 1)
>> +
>> +ifeq ($(CONFIG_GCOV_FORMAT_3_4),y)
>> +  cc-ver := 0304
>> +else ifeq ($(CONFIG_GCOV_FORMAT_4_7),y)
>> +  cc-ver := 0407
>> +else
>> +  cc-ver := $(call cc-version)
>> +endif
>> +
>>  obj-$(CONFIG_GCOV_KERNEL) := base.o fs.o
>> -obj-$(CONFIG_GCOV_KERNEL) += $(call cc-ifversion, -lt, 0407, gcc_3_4.o)
>> -obj-$(CONFIG_GCOV_KERNEL) += $(call cc-ifversion, -ge, 0407, gcc_4_7.o)
>> +
>> +ifeq ($(call if-lt, $(cc-ver), 0407),1)
>> +  obj-$(CONFIG_GCOV_KERNEL) += gcc_3_4.o
>> +else
>> +  obj-$(CONFIG_GCOV_KERNEL) += gcc_4_7.o
>> +endif
>>
>>
>> -- 
>> Peter Oberparleiter
>> Linux on System z Development - IBM Germany
>>
> 


-- 
Peter Oberparleiter
Linux on System z Development - IBM Germany

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