[Bug gas/27075] New: [MIPS] Gas adds useless delay-slot nops to r6 compact branches, which have no delay slots

[positron at gnu dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=27075

            Bug ID: 27075
           Summary: [MIPS] Gas adds useless delay-slot nops to r6 compact
                    branches, which have no delay slots
           Product: binutils
           Version: 2.35.1
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gas
          Assignee: unassigned at sourceware dot org
          Reporter: positron at gnu dot org
  Target Milestone: ---

Created attachment 13052
  --> https://sourceware.org/bugzilla/attachment.cgi?id=13052&action=edit
interaction showing my test case and the tool output

While adding support for MIPS release 6 to my software I noticed a
bizarre behaviour related to compact branch instructions, in particular
conditionals.

Gas adds a useless nop instruction (sll $0,$0,0x0) after most but not
all compact branches, unless the instruction following the branch looks
like a "plausible" delay-slot instructions (more details in the test
case).

For example, this:

        .set arch=mips32r6
foo:
        bnezc $3, foo
        bltzc $3, foo
        bgtzc $3, foo
        blezc $3, foo
        bgezc $3, foo


disassembles to:

00000000 <foo>:
   0:   f87fffff        bnezc   $3,0 <foo>
   4:   00000000        sll     $0,$0,0x0
   8:   5c63fffd        bltzc   $3,0 <foo>
   c:   00000000        sll     $0,$0,0x0
  10:   5c03fffb        bgtzc   $3,0 <foo>
  14:   00000000        sll     $0,$0,0x0
  18:   5803fff9        blezc   $3,0 <foo>
  1c:   00000000        sll     $0,$0,0x0
  20:   5863fff7        bgezc   $3,0 <foo>
        ...

This, while not technically assembling incorrect code, does not make
sense: the instruction following the compact branch is *not* executed
when the branch is taken, which is the entire point of compact branches.
This should be independent from what instruction follows the branch,
since at least as far as I understand there is no restriction on what
instructions are allowed in that position.

The balc instruction is not affected.
The bc instruction is not affected.
The jalrc instruction is not affected.
The jic instruction is not affected.

Every conditional compact branch I have tried *is* affected.  I have not
tested the linking versions, but on the other hand every conditional
branch seems affected: on zero, on sign, on magnitude.  This includes
for example beqzc, bltzc, bgtzc, bltc.

I am using inline asm from GNU C, from which it is not practical to work
around the misbehaviour with
  .set noreorder
, since the last instruction before I switch back to
  .set reorder
will be "completed" with a useless nop, unless the following instruction
appears to be suitable to fill the (not actually existing) delay slot.

I am attaching a distilled test case along with what the tools show me.

Tested with Binutils 2.35.1 (on a cross configuration to big-endian
32-bit MIPS), compiled with crosstool but using stock source tarballs
downloaded from ftp.gnu.org, with no patches.

Binutils 2.35.1, compiled via crosstool-ng with no patches:
m6[luca@moore ~/repos/jitter/_build/cross-mips32r6]$ mips-unknown-linux-gnu-as
--version
GNU assembler (crosstool-NG 1.23.0.942_5fbcd4b) 2.35.1
Copyright (C) 2020 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or later.
This program has absolutely no warranty.
This assembler was configured for a target of `mips-unknown-linux-gnu'.

m6[luca@moore ~/repos/jitter/_build/cross-mips32r6]$ mips-unknown-linux-gnu-gcc
--version
mips-unknown-linux-gnu-gcc (crosstool-NG 1.23.0.942_5fbcd4b) 10.2.0
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

m6[luca@moore ~/repos/jitter/_build/cross-mips32r6]$ mips-unknown-linux-gnu-gcc
-v -c testcase-beqzc.s
Using built-in specs.
COLLECT_GCC=mips-unknown-linux-gnu-gcc
Target: mips-unknown-linux-gnu
Configured with:
/home/luca/cross-scratch/mips32r6/.build/mips-unknown-linux-gnu/src/gcc/configure
--build=x86_64-build_pc-linux-gnu --host=x86_64-build_pc-linux-gnu
--target=mips-unknown-linux-gnu --prefix=/home/luca/usr-cross-mips32r6
--with-sysroot=/home/luca/usr-cross-mips32r6/mips-unknown-linux-gnu/sysroot
--enable-languages=c,c++ --with-arch=mips32r6 --with-abi=32 --with-float=hard
--with-pkgversion='crosstool-NG 1.23.0.942_5fbcd4b' --enable-__cxa_atexit
--disable-libmudflap --disable-libgomp --disable-libssp --disable-libquadmath
--disable-libquadmath-support --disable-libsanitizer --disable-libmpx
--with-gmp=/home/luca/cross-scratch/mips32r6/.build/mips-unknown-linux-gnu/buildtools
--with-mpfr=/home/luca/cross-scratch/mips32r6/.build/mips-unknown-linux-gnu/buildtools
--with-mpc=/home/luca/cross-scratch/mips32r6/.build/mips-unknown-linux-gnu/buildtools
--with-isl=/home/luca/cross-scratch/mips32r6/.build/mips-unknown-linux-gnu/buildtools
--disable-lto --with-host-libstdcxx='-static-libgcc
-Wl,-Bstatic,-lstdc++,-Bdynamic -lm' --enable-threads=posix
--enable-target-optspace --enable-plugin --disable-nls --disable-multilib
--with-local-prefix=/home/luca/usr-cross-mips32r6/mips-unknown-linux-gnu/sysroot
--enable-long-long
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 10.2.0 (crosstool-NG 1.23.0.942_5fbcd4b) 
COLLECT_GCC_OPTIONS='-v' '-c' '-march=mips32r6' '-mabi=32' '-mhard-float'
'-mllsc' '-mips32r6' '-mnan=2008' '-mno-shared' '-EB'

/home/luca/usr-cross-mips32r6/lib/gcc/mips-unknown-linux-gnu/10.2.0/../../../../mips-unknown-linux-gnu/bin/as
-v -EB -mips32r6 -O1 -mabi=32 -march=mips32r6 -mnan=2008 -mno-shared
-mhard-float -KPIC -o testcase-beqzc.o testcase-beqzc.s
GNU assembler version 2.35.1 (mips-unknown-linux-gnu) using BFD version
(crosstool-NG 1.23.0.942_5fbcd4b) 2.35.1
COMPILER_PATH=/home/luca/usr-cross-mips32r6/libexec/gcc/mips-unknown-linux-gnu/10.2.0/:/home/luca/usr-cross-mips32r6/libexec/gcc/mips-unknown-linux-gnu/10.2.0/:/home/luca/usr-cross-mips32r6/libexec/gcc/mips-unknown-linux-gnu/:/home/luca/usr-cross-mips32r6/lib/gcc/mips-unknown-linux-gnu/10.2.0/:/home/luca/usr-cross-mips32r6/lib/gcc/mips-unknown-linux-gnu/:/home/luca/usr-cross-mips32r6/lib/gcc/mips-unknown-linux-gnu/10.2.0/../../../../mips-unknown-linux-gnu/bin/
LIBRARY_PATH=/home/luca/usr-cross-mips32r6/lib/gcc/mips-unknown-linux-gnu/10.2.0/:/home/luca/usr-cross-mips32r6/lib/gcc/mips-unknown-linux-gnu/10.2.0/../../../../mips-unknown-linux-gnu/lib/:/home/luca/usr-cross-mips32r6/mips-unknown-linux-gnu/sysroot/lib/:/home/luca/usr-cross-mips32r6/mips-unknown-linux-gnu/sysroot/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-c' '-march=mips32r6' '-mabi=32' '-mhard-float'
'-mllsc' '-mips32r6' '-mnan=2008' '-mno-shared' '-EB'

Thank you very much for your work on the Binutils.
Best regards,

-- 
Luca Saiu
* My personal web site:  http://ageinghacker.net
* Jitter:                http://ageinghacker.net/projects/jitter
* GNU epsilon:           http://www.gnu.org/software/epsilon

I support everyone's freedom of mocking any opinion or belief, no
matter how deeply held, with open disrespect and the same unrelented
enthusiasm of a toddler who has just learned the word "poo".

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