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Re: [PATCH] Added information about inline assembler in stack calculations (.su files)


On 12/7/18 4:35 PM, Segher Boessenkool wrote:
> Hi!
> 
> On Fri, Dec 07, 2018 at 07:51:35AM +0000, Niklas DAHLQUIST wrote:
>> On 12/1/18 1:15 AM, Jeff Law wrote:
>>> One could argue that allocating stack space inside an ASM is a really
>>> bad idea.  Consider things like dwarf debugging and unwind tables.  If
>>> you're allocating stack inside an ASM that stuff is going to be totally
>>> wrong.
>>>
>>> So I think my question before moving forward with something like this is
>>> whether or not it makes sense at all to bother dumping data for a
>>> scenario that we'd probably suggest developers avoid to begin with.
>>
>> The purpose of the patch is to notify when the reported stack usage might be
>> incorrect. Even if it's bad practice to alter stack in asm, there are 
>> use cases
>> in the embedded world that makes sense. A notable common use case is 
>> FreeRTOS
>> task switch using ARM "naked" attribute and inline asm, which reports "0 
>> static",
>> which gives a faulty stack usage. We have considered the other option to
>> report a warning for these cases, but that alternative hasn't appealed 
>> to us.
> 
> Would that work well?  Only warn for naked functions?  It would work
> better for all users that do *not* mess with the stack in their asm ;-)
What I'm questioning is whether or not this is at all useful.  ie, if
I've written a something like task switching in C+asms, then I would
fully expect any  data related to stack usage in that function to be
totally bogus.  Telling me it's bogus in the assembly output really
isn't of value.  It's telling me something I should clearly already know.

And in the common case of an asm that doesn't mess with the stack at
all, the stack usage info is valid and warning me that it may not be is
just a huge annoyance.

jeff


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