riscv64/x390: add *_overflow

[ghostway0]

currently a draft and only riscv64

[ghostway0]

@bjorn3 can you take a look? also, I haven't found umul equivalent (or mul with zext*) in s390x. do you know what are their names?

[uweigand]

Hi @ghostway0, a few comments on the s390x part:

• All the new instruction rules you added seem to provide only a single return, the overflow bit. However, my understanding is that smul_overflow and all the other overflow instructions are defined to have two returns, the low-part result and the overflow bit. I think you'll need to use some form of with_flags to construct the pair of results (like x86 and aarch64 already do).
• You're simply re-using the same instructions used for the "normal" operation (add/sub/mul) also for the overflow operation. That is correct for 32-bit and 64-bit operations, but not for 8-bit and 16-bit operations. The reason is that the 390x ISA does not actually have any 8-bit or 16-bit arithmetic instructions, so we simply use the 32-bit version also for 8-bit and 16-bit operations. That provides the correct (low-part) result, but any overflow indication would be incorrect.
• There is no unsigned-multiply instruction with overflow indication on our platform. What other compilers do is to use the 32x32->64 or 64x64->128 bit wide multiply instruction, and check whether the high-part of the output is zero.