https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108031
--- Comment #6 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jeff Law <[email protected]>: https://gcc.gnu.org/g:0cfd3de946cedafc0948fe4fde5a015c63b8993c commit r17-3276-g0cfd3de946cedafc0948fe4fde5a015c63b8993c Author: Jeff Law <[email protected]> Date: Thu Aug 13 21:22:57 2026 -0600 [RISC-V][PR target/108031] Adjust cost of LO_SUM expression slightly So this is the second part to pr108031. Originally I thought this was going to be related to Shreya's work on fixing various cost model goofs in the RISC-V backend and that if we fixed the cost modeling that the right things would just happen. Essentially what I've had her focused on is cases where we have a single cycle ALU insn, but the reported cost is anything but 4. Lots of these issues have been clustered around embedded extensions, complex RTL for bitmanip instructions, symbolic addresses, etc. I figured the costing issue with 108031 would be related. But it'd been a while since I did that very quick triage, so I went back and took a closer look at the behavior inside CSE; that's when it became clear that the LO_SUM expression and related value equivalent were reporting the same cost. In that scenario CSE (reasonably) chooses to keep things as-is to avoid gratuitous IL changes. So we need the LO_SUM expression to cost higher than a simple PLUS, even though they both collapse into addi/add insns. It's not as crazy at it might seem as anytime we can get rid of the LO_SUM form, we're likely going to get rid of the HIGH expression as well. Other ports cost symbolics much higher and the RISC-V port may want to follow suit at some point, but for now we can just add a single unit to the LO_SUM expression cost and all the right things happen -- hopefully with no fallout. This has been tested on riscv32-elf and riscv64-elf. Bootstraps on the K3 and c920 will run overnight, but I don't expect any issues. Pushing to the trunk. PR target/108031 gcc/ * config/riscv/riscv.cc (riscv_rtx_costs): Bump the cost of a LO_SUM by one unit to encourage use of related values instead of the symbolic form. gcc/testsuite * gcc.target/riscv/pr108031-2.c: New test.
