FW: [PATCH] [MIPS] microMIPS gcc support

[Richard Sandiford]

"Maciej W. Rozycki" <macro@codesourcery.com> writes:
> On Wed, 23 Jan 2013, Richard Sandiford wrote:
>> > Index: config/mips/gnu-user.h
>> > ===================================================================
>> > --- config/mips/gnu-user.h	(revision 195351)
>> > +++ config/mips/gnu-user.h	(working copy)
>> > @@ -137,3 +137,12 @@ extern const char *host_detect_local_cpu (int argc
>> >  #define ENDFILE_SPEC \
>> >    GNU_USER_TARGET_MATHFILE_SPEC " " \
>> >    GNU_USER_TARGET_ENDFILE_SPEC
>> > +
>> > +#undef SUBTARGET_OVERRIDE_OPTIONS
>> > +#define SUBTARGET_OVERRIDE_OPTIONS                              \
>> > +do {                                                            \
>> > +  /* microMIPS PLT entries are non-microMIPS.  */               \
>> > +  TARGET_INTERLINK_COMPRESSED = 1;                              \
>> > +} while (0)
>> > +
>> > +
>> 
>> Hmm, that sounds like a reason to set TARGET_INTERLINK_UNCOMPRESSED
>> rather than TARGET_INTERLINK_COMPRESSED.  I.e. we need to avoid direct
>> branches from microMIPS code to standard PLTs.
>> 
>> But that means that microMIPS code can't even jump directly to functions
>> that have a micromips attribute when the call might go via a PLT.
>> TARGET_INTERLINK_(UN)COMPRESSED doesn't cover that case.  I think instead
>> we need to handle it directly in mips_function_ok_for_sibcall,
>> keyed off TARGET_ABICALLS_PIC0.  Specific suggestion below.
> [...]
>> >    /* We can't do a sibcall if the called function is a MIPS16 function
>> >       because there is no direct "jx" instruction equivalent to "jalx" to
>> >       switch the ISA mode.  We only care about cases where the sibling
>> >       and normal calls would both be direct.  */
>> >    if (decl
>> > -      && mips_use_mips16_mode_p (decl)
>> > +      && ((compression_mode & (MASK_MIPS16 | MASK_MICROMIPS)) != 0)
>> >        && const_call_insn_operand (XEXP (DECL_RTL (decl), 0), VOIDmode))
>> >      return false;
>> >  
>> > -  /* When -minterlink-mips16 is in effect, assume that non-locally-binding
>> > -     functions could be MIPS16 ones unless an attribute explicitly tells
>> > +  /* When -minterlink-compressed is in effect, assume that non-locally-binding
>> > +     functions could be MIPS16 or microMIPS unless an attribute explicitly tells
>> >       us otherwise.  */
>> > -  if (TARGET_INTERLINK_MIPS16
>> > +  if (TARGET_INTERLINK_COMPRESSED
>> >        && decl
>> >        && (DECL_EXTERNAL (decl) || !targetm.binds_local_p (decl))
>> > -      && !mips_nomips16_decl_p (decl)
>> > +      && (compression_mode & MASK_MICROMIPS) == 0
>> > +      && (compression_mode & MASK_MIPS16) == 0
>> >        && const_call_insn_operand (XEXP (DECL_RTL (decl), 0), VOIDmode))
>> >      return false;
>> 
>> should be something like:
>> 
>>   /* Direct Js are only possible to functions that use the same ISA encoding.
>>      There is no JX counterpart of JALX.  */
>>   if (decl
>>       && const_call_insn_operand (XEXP (DECL_RTL (decl), 0), VOIDmode)
>>       && mips_call_may_need_jalx_p (decl))
>>     return false;
>> 
>> with:
>> 
>> /* Return true if a call to DECL may need to use JALX.  */
>> 
>> static bool
>> mips_call_may_need_jalx_p (decl)
>> {
>>   /* If the current translation unit would use a different mode for DECL,
>>      assume that the call needs JALX.  */
>>   if (mips_get_compress_mode (decl) != TARGET_COMPRESSION)
>>     return true;
>> 
>>   /* mips_get_compress_mode is always accurate for locally-binding
>>      functions in the current translation unit.  */
>>   if (!DECL_EXTERNAL (decl) && targetm.binds_local_p (decl))
>>     return false;
>> 
>>   /* PLTs use the standard encoding, so calls that might go via PLTs
>>      must use JALX.  */
>>   if (TARGET_COMPRESSED
>>       && TARGET_ABICALLS_PIC0
>>       && DECL_EXTERNAL (decl)
>>       && !targetm.binds_local_p (decl))
>>     return true;
>> 
>>   /* When -minterlink-compressed is in effect, assume that functions
>>      could use a different encoding mode unless an attribute explicitly
>>      tells us otherwise.  */
>>   if (TARGET_INTERLINK_COMPRESSED)
>>     {
>>       if (!TARGET_COMPRESSION && mips_get_compress_off_flags (decl) == 0)
>>         return true;
>>       if (TARGET_COMPRESSION && mips_get_compress_on_flags (decl) == 0)
>>         return true;
>>     }
>> 
>>   return false;
>> }
>> 
>> The TARGET_ABICALLS_PIC0 case should deal with the gnu-user.h comment above.
>
>  How about instead of complicating this we simply add support for 
> microMIPS encoding in the PLT?  I think I should be able to squeeze out 
> some time next week to dust off and retest the binutils patch I've had 
> pending far too long now.  This way we won't have to maintain separate 
> cases where tail calls may or may not be made via the PLT.
>
>  Note that we need that support sooner or later anyway due to the prospect 
> of pure-microMIPS processors.

Just so I know: what does the PLT patch do for external functions
that are jumped to by both microMIPS and non-microMIPS code?

Richard