Hello. 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:
--8<---------------cut here---------------start------------->8---
.set arch=mips32r6
foo:
bnezc $3, foo
bltzc $3, foo
bgtzc $3, foo
blezc $3, foo
bgezc $3, foo
--8<---------------cut here---------------end--------------->8---
disassembles to:
--8<---------------cut here---------------start------------->8---
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 $3,0 <foo>
...
--8<---------------cut here---------------end--------------->8---
This, while not technically assembling incorrect code, does not make
sense: the instruction following the compact branch is *not* executed
when the branch is taken, which is the entire point of compact branches.
This should be independent from what instruction follows the branch,
since at least as far as I understand there is no restriction on what
instructions are allowed in that position.
The balc instruction is not affected.
The bc instruction is not affected.
The jalrc instruction is not affected.
The jic instruction is not affected.
Every conditional compact branch I have tried *is* affected. I have not
tested the linking versions, but on the other hand every conditional
branch seems affected: on zero, on sign, on magnitude. This includes
for example beqzc, bltzc, bgtzc, bltc.
I am using inline asm from GNU C, from which it is not practical to work
around the misbehaviour with
.set noreorder
, since the last instruction before I switch back to
.set reorder
will be "completed" with a useless nop, unless the following instruction
appears to be suitable to fill the (not actually existing) delay slot.
I am attaching a distilled test case along with what the tools show me.
Tested with Binutils 2.35.1 (on a cross configuration to big-endian
32-bit MIPS), compiled with crosstool but using stock source tarballs
downloaded from ftp.gnu.org, with no patches.
Binutils 2.35.1, compiled via crosstool-ng with no patches:
m6[luca@moore ~/repos/jitter/_build/cross-mips32r6]$ 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".
interaction
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