[PATCH][mips] Introduce '.set noase' to the mips assembler.

[Egeyar Bagcioglu]

Hello,

I would like to propose the attached tiny patch, together with the 
lengthy explanation of its use case. Please review.

Currently, the mips assembler keeps, as a part of its runtime state, 
information regarding the accepted ISA and ASEs (Application Specific 
Extensions). It is possible to change the ISA or allowed extensions on 
the fly. It is also possible to use ".set push" to save the state 
regarding the accepted instructions, change them, and pop the previously 
pushed state afterwards.

Linux kernel takes advantage of this situation in its mips inline 
assembly functions. The assembler is asked to push into the stack the 
information of the target platform. The ISA is then set to mips3, which 
is the strictest, to make sure that the assembler generates an error if 
the inline assembler is not accepted by all mips platforms. At the end 
of the inline assembly, the previous state of the assembler is restored 
with a ".set pop":

.set push
.set mips3Â Â Â Â Â  #the base ISA
<assembly>
.set pop

The above usage works mostly, when no ASEs are enabled or when the 
enabled ASEs are accepted in mips3 due other settings (such as micromips 
being enabled, etc.). If, however, a conflicting ASE is enabled, the 
switch to mips3 causes the assembler to generate a warning per file 
because mips3 does not support that given ASE. While building a big 
project such as the kernel, one warning per file  with inline assembly 
means several hundreds of warnings in total.

I have considered different ways to solve this issue. If we implicitly 
disable the unsupported ASEs upon switching to a new ISA, we lose 
backwards compatibility. Any project which is -for whatever reason- 
generating some extension instructions that are not supported by the 
given ISA would start getting errors from the assembler. If we decide to 
turn off the warnings under whatever condition, the assembler would 
start accepting unsupported extension instructions without even a 
warning. This would, for example, undermine the kernel's use-case. A 
more-user-friendly option could be delaying the warning. We would not 
warn when a conflicting ISA is set. Instead, we would store this 
information, check the following instructions and only warn if/when we 
see the first instruction using a conflicting ASE. However, I believe 
the overhead of this approach could not be justified by the need.

The change I propose is to add a new ".set noase" directive to disable 
all the ASEs. This pseudo-op would be beneficial primarily within .set 
push/pop pairs of inline assembly functions as used by the kernel, by 
explicitly disabling all 19 extensions of mips architecture and avoiding 
the related warnings. An example usage of it would transform the inline 
assembly functions into the following format:

.set push
.set noase
.set mips3Â Â Â  #the base ISA
<assembly>
.set pop


As .set push/pop pairs also saves and restores the state of enabled 
ASEs, this would indeed serve the purpose of limiting the generation of 
extension instructions within that pair, without permanently changing 
which instructions are allowed. Having said that, the execution of this 
directive does not depend on the .set push/pop pairs. Therefore, it can 
also be used separately.

I do not know how likely it is for the kernel or other projects to use 
this new directive as it would depend on a very new binutils patch. 
Although, it might be applied together with a macro controlling its 
usage. Either way, I still see value in assembler supporting such a usage.

As it is new, this directive does not interfere with the existing 
regression tests. Therefore, you'll see a new test case in the patch as 
well.

The suggested gas/ChangeLog is
2019-09-26  Egeyar Bagcioglu  <egeyar.bagcioglu@oracle.com>

 Â Â Â Â Â Â Â  * config/tc-mips.c (parse_code_option): Introduce ".set/.module
 Â Â Â Â Â Â Â  noase" pseudo-op to disable all Application Specific Extensions.
 Â Â Â Â Â Â Â  * gas/testsuite/gas/mips/ase-errors-5.l: New test.
 Â Â Â Â Â Â Â  * gas/testsuite/gas/mips/ase-errors-5.s: New test source.
 Â Â Â Â Â Â Â  * gas/testsuite/gas/mips/mips.exp: Run the new test.


Please review and let me know what you think.

Thanks,
Egeyar
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