Hi, I think gcc is relying on undefined behaviour with the vcompress instruction. Unfortunately my test case isn't reproducing on mainline, but gcc looks to use the fields between the last mask selected field and vl while setting tail agnostic.
This thread explains how vcompress is different in that the tail starts after the last mask selected field: https://github.com/riscvarchive/riscv-v-spec/issues/796 There was a bug in QEMU that I just fixed that prevented the all 1s tail agnostic option (rvv_ta_all_1s) from poisoning these bits: https://lists.nongnu.org/archive/html/qemu-riscv/2024-10/msg00561.html With that fix, I see problems until I modify the previous setvli from ta to tu. I think 9aabf81f40f0 ("RISC-V: Optimize permutation codegen with compress") is one place we need to set tail undisturbed, but my fail is from an earlier checkout so there must be another issue in the tree. I presume the fix is to force all these vcompress cases to set tail undisturbed. Thanks, Anton
