[RFC] Only NOP in delayed branches in .MIPS.stubs section

[Maciej W. Rozycki]

Hi Maksim,

> I work with mips32 based SoC, which has some restrictions (errata). One of
> them - only the NOP instruction in delay slot is allowed.

 Ouch!

> Mainly, this restriction is resolved easily by passing -fno-delayed-branch
> flag to compiller and by patching .noreorder sections in assembler code.
> But there is a problem with .MIPS.stubs section - ELF backend linker
> generates code like this:

 What about the PLT?

> For now, to resolve this, I use my own patch which is just a dirty hack
> (bellow, at the end of the message). I'm not a GNU tools expert, but, in my
> humble opinion, it would be great to get the same result just by passing
> -fno-delayed-branch flag to compiler.

 That's surely doable.

> I would like to create more proper patch, but my main difficulty is that I
> can not found a right way to detect if -fno-delayed-branch flag is used
> (was used in as). So, I need advice to resolve this. I would be very
> thankfull for that.

 First you need to pass the GCC driver this flag in the link stage as well 
(conventionally among the flags set with the $(LDFLAGS) variable in 
Makefile).  Then you can modify GCC driver specs (see the output from `gcc 
-dumpspecs' and the `link' recipe there) to have the flag set a 
corresponding linker flag in LD invocation, that you need to define.

 We have a few MIPS-specific linker flags already defined in 
ld/emultempl/mipself.em (see PARSE_AND_LIST_* variables), and your new 
option will go there (you'll need both a positive and a negative form, so 
that overriding both ways is possible, and as you can see the convention 
for linker flags is to drop the `f' or `m' prefix and use a double leading 
dash).

> And more general question - does it make sense to develop correct patch for
> that at all? Or this is minor specific problem that everyone should resolve
> himself for his specific situation?

 We do accept properly formed submissions for hardware errata workarounds.  
See: <https://sourceware.org/gdb/wiki/ContributionChecklist> for how a 
proper submission is supposed to be made and if you are unsure about 
anything, then please do not hesitate to ask.

 Hope this helps.

  Maciej