For a one-time use for a small program or couple functions, you don't need to install anything locally.
Use Matt Godbolt's compiler explorer site, https://godbolt.org/, which has GCC and clang for various ISAs including MIPS and x86-64, and some other compilers.
Note that the compiler explorer by default filters directives so you can just see the instructions, leaving out stuff like alignment, sections, .globl, and so on. (For a function with no global / static data, this is actually fine, especially when you just want to use a compiler to make an example for you. The default section is .text anyway, if you don't use any directives.)
Most people that want MIPS asm for homework are using SPIM or MARS, usually without branch-delay slots. (Unlike real MIPS, so you need to tweak the compiler to not take advantage of the next instruction after a branch running unconditionally, even when it's taken.) For GCC, the option is -fno-delayed-branch - that will fill every delay slot with a NOP, so the code will still run on a real MIPS. You can just manually remove all the NOPs.
There may be other tweaks needed, like MARS may require you to use jr $31 instead of j $31, Tweak mips-gcc output to work with MARS without using a script. And of course I/O code will have to be implemented using MARS's toy system calls, not jal calls to standard library functions like printf or std::ostream::operator<<. You can usefully compile (and hand-tweak) asm for manipulating data, like multiplying integers or summing or reversing an array, though.
Unfortunately GCC doesn't have an option to use register names like $a0 instead of $r. For PowerPC there's -mregnames to use r1 instead of 1, but no similar option for MIPS to use "more symbolic" reg names.
int maybe_square(int num) {
if (num>0)
return num;
return num * num;
}
On Godbolt with GCC 5.4 -xc -O3 -march=mips32r2 -Wall -fverbose-asm -fno-delayed-branch
-xc compiles as C, not C++, because I find that more convenient than flipping between the C and C++ languages in the dropdown and having the site erase my source code.
-fverbose-asm comments the asm with C variable names for the destination and sources. (In optimized code that's often an invented temporary, but not always.)
-O3 enables full optimization, because the default -O0 debug mode is a horrible mess for humans to read. Always use at least -Og if you want to look at the code by hand and see how it implements the source. How to remove "noise" from GCC/clang assembly output?. You might also use -fno-unroll-loops, and -fno-tree-vectorize if compiling for an ISA with SIMD instructions.
This uses mul instead of the classic MIPS mult + mflo, thanks to the -march= option to tell GCC we're compiling for a later MIPS ISA, not whatever the default baseline is. (Perhaps MIPS I aka R2000, -march=mips1)
See also the GCC manual's section on MIPS target options.
# gcc 5.4 -O3
square:
blez $4,$L5
nop
move $2,$4 # D.1492, num # retval = num
j $31 # jr $ra = return
nop
$L5:
mul $2,$4,$4 # D.1492, num, num # retval = num * num
j $31 # jr $ra = return
nop
Or with clang, use -target mips to tell it to compile for MIPS. You can do this on your desktop; unlike GCC, clang is normally built with multiple back-ends enabled.
From the same Godbolt link, clang 10.1 -xc -O3 -target mips -Wall -fverbose-asm -fomit-frame-pointer. The default target is apparently MIPS32 or something like that for clang. Also, clang defaults to enabling frame pointers for MIPS, making the asm noisy.
Note that it chose to make branchless asm, doing if-conversion into a conditional-move to select between the original input and the mul result. Unfortunately clang doesn't support -fno-delayed-branch; maybe it has another name for the same option, or maybe there's no hope.
maybe_square:
slti $1, $4, 1
addiu $2, $zero, 1
movn $2, $4, $1 # conditional move based on $1
jr $ra
mul $2, $2, $4 # in the branch delay slot
In this case we can simply put the mul before the jr, but in other cases converting to no-branch-delay asm is not totally trivial. e.g. branch on a loop counter before decrementing it can't be undone by putting the decrement first; that would change the meaning.
Register names:
Compilers use register numbers, not bothering with names. For human use, you will often want to translate back. Many places online have MIPS register tables that show how $4..$7 are $a0..$a3, $8 .. $15 are $t0 .. $t7, etc. For example this one.
