Re: [MIPS] Gas adds useless delay-slot nops to r6 compact branches, which have no delay slots

[Luca Saiu]

On 2020-12-15 at 18:03 +0100, Luca Saiu wrote:

> This, while not technically assembling incorrect code, does not make
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

One more remark: I was not able to reproduce this behaviour with compact
branches, but it is possible that in some cases Gas' rewrite machinery
might actually move a previous instruction past the branch instead of
inserting a nop.  That would in fact be incorrect code, with compact
branches.

For example (using a traditional MIPS branch with delay slot) this:
--8<---------------cut here---------------start------------->8---
        addu  $10, $11, $12
        beq   $13, $14, foo
--8<---------------cut here---------------end--------------->8---
may turn into:
--8<---------------cut here---------------start------------->8---
   0:   11aeffff        beq     $13,$14,0x0
   4:   016c5021        addu    $10,$11,$12
--8<---------------cut here---------------end--------------->8---

If that happened with compact branches it would be a disaster.

I am adding a report to Bugzilla.

-- 
Luca Saiu
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