gcc - GNU Compiler Collection (original) (raw)

[RISC-V] Split X eq/ne C where -C is a small constantHEADtrunkmaster

This was something I found while analyzing paths forward for a patch from Daniel. Amazingly, RISC-V does not have anything like a setCC style insn that compares a register against a constant. Instead we negate the constant and add it to the source value. That gives us zero (equal) or nonzero (not equal). We follow that with a snez/seqz to give us 0/1 like other setCC style instructions. If we have a 3 or more insns that ultimately combine into something like: (set (dest) (eq (srcreg) (const_int)) We can use a define_split to rewrite that into a two instruction sequence which is a small win. I'd suspected there was some value in this kind of splitter for a while, but never had a testcase that could actually be improved. I wrote the splitter and tested with Daniel's code, but more importantly, once I had the basic splitter working, I could do a before/after comparison and look for differences which I was able to find. While the testcase came from 502.gcc, it's not hot at all. But it does clearly show how the splitter can improve code. Tested on riscv32-elf and riscv64-elf. Bootstraps on the K3 and c920 are in flight. I'll wait for the bootstrap/regression tests as well as the pre-commit CI testing before moving forward. gcc/ * config/riscv/riscv.md (splitter for equality test): New splitter. gcc/testsuite/ * gcc.target/riscv/test-equal.c: New test.

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