<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Fri, Nov 20, 2015 at 11:02 PM, Simon Atanasyan <span dir="ltr"><<a href="mailto:simon@atanasyan.com" target="_blank">simon@atanasyan.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><span>On Sat, Nov 21, 2015 at 9:28 AM, Rui Ueyama <<a href="mailto:ruiu@google.com" target="_blank">ruiu@google.com</a>> wrote:<br>
> On Fri, Nov 20, 2015 at 10:13 PM, Simon Atanasyan <<a href="mailto:simon@atanasyan.com" target="_blank">simon@atanasyan.com</a>><br>
> wrote:<br>
>><br>
</span><div><div>>> In case of MIPS O32 ABI we have to find a matching R_MIPS_LO16<br>
>> relocation to calculate R_MIPS_HI16 one because R_MIPS_HI16 uses<br>
>> combined addend (AHI << 16) + (short)ALO where AHI is original<br>
>> R_MIPS_HI16 addend and ALO is addend of the matching R_MIPS_LO16<br>
>> relocation [1]. There are two methods to do matching and R_MIPS_HI16<br>
>> calculation.<br>
>><br>
>> Method A:<br>
>> 1. Postpone R_MIPS_HI16 relocation calculation and record its arguments.<br>
>> 2. When R_MIPS_LO16 is found, iterate over recorded R_MIPS_HI16<br>
>> relocations, calculate combined addend and apply relocations.<br>
>> 3. At the end check orphaned (without R_MIPS_LO16 pair) R_MIPS_HI16<br>
>> relocations, show warnings and apply them with zero addend.<br>
>><br>
>> Method B:<br>
>> 1. Each time we have found R_MIPS_HI16 relocation, iterate remaining<br>
>> relocations list to find matching R_MIPS_LO16.<br>
>> 2. Calculate combined adddend and apply relocation or show warning if<br>
>> the R_MIPS_LO16 is not found.<br>
>><br>
>> Method A requires some sort of container to keep postponed HI16<br>
>> relocations. If we add the container to the `MipsTargetInfo` class we<br>
>> will be able to hide all this unusual scheme inside MIPS specific code<br>
>> and will not need to perform LO16 lookups. But the `MipsTargetInfo`<br>
>> becomes stateful.<br>
>><br>
>> Method B keeps the `MipsTargetInfo` stateless but requires forward<br>
>> LO16 lookup for each HI16 relocation and requires to provide an<br>
>> interface for such lookup to the `MipsTargetInfo`.<br>
>><br>
>> Sure we can implement each of these methods somewhere in the<br>
>> `InputSectionBase` class under `if (MIPS)` statements.<br>
>><br>
>> Any opinions about the best method / approach?<br>
><br>
><br>
> If I understand that spec correctly, an R_MIPS_HI16 should immediately be<br>
> followed by an R_MIPS_LO16. Can't you use that property? It doesn't seem to<br>
> me that you really have to search and pair up HI16 and LO16 relocations.<br>
<br>
</div></div>It is a question what the ABI authors did mean by the "R_MIPS_HI16<br>
must have an associated R_MIPS_LO16 entry immediately following it"<br>
phrase. In fact you can get from a compiler this code:<br>
<br>
lui $t0,%hi(sym1+4) # R_MIPS_HI16<br>
lui $t0,%hi(sym1+8) # R_MIPS_HI16<br>
lui $t0,%hi(sym1+12) # R_MIPS_HI16<br>
addi $t0,$t0,%lo(sym1+16) # R_MIPS_LO16<br></blockquote><div><br></div><div>The first two relocations don't conform to the standard because there are no corresponding LO16 relocations, no?</div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
and even such code:<br>
<br>
lui $t0,%hi(sym1) # R_MIPS_HI16 on sym1<br>
lui $t0,%hi(sym2) # R_MIPS_HI16 on sym2<br>
addi $t0,$t0,%lo(sym1) # R_MIPS_LO16 on sym1<br>
addi $t0,$t0,%lo(sym2) # R_MIPS_LO16 on sym2<br></blockquote><div><br></div><div>Hmm, isn't this a violation of the ABI? My interpretation of "[e]ach relocation type of R_MIPS_HI16 must have an associated R_MIPS_LO16 entry immediately following it in the list of relocations" is not ambiguous to allow them. Is there any chance to fix the compiler? (I guess there isn't, though.)</div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
fortunately I have never seen such code:<br>
<br>
lui $t0,%hi(sym1+12) # R_MIPS_HI16<br>
... other type (except HI16 / LO16) of relocation here<br>
addi $t0,$t0,%lo(sym1+16) # R_MIPS_LO16<br>
<span><font color="#888888"><br>
--<br>
Simon Atanasyan<br>
</font></span></blockquote></div><br></div></div>