Committed to trunk
On Mon, Apr 24, 2023 at 11:06 AM juzhe.zh...@rivai.ai <juzhe.zh...@rivai.ai> wrote: > > Adding earlyclobber is to make dest operand do not overlap with source > operand. > For example: > for gather load, vluxei.v v8,(a5),v8 is illegal according to RVV ISA. > GCC is using same way as LLVM which is also adding earlyclobber for modeling > disabling overlap between dest and source operand. > > > > juzhe.zh...@rivai.ai > > From: Jeff Law > Date: 2023-04-22 04:36 > To: juzhe.zhong; gcc-patches > CC: kito.cheng > Subject: Re: [PATCH] RISC-V: Fine tune gather load RA constraint > > > On 3/13/23 02:28, juzhe.zh...@rivai.ai wrote: > > From: Ju-Zhe Zhong <juzhe.zh...@rivai.ai> > > > > For DEST EEW < SOURCE EEW, we can partial overlap register > > according to RVV ISA. > > > > gcc/ChangeLog: > > > > * config/riscv/vector.md: Fix RA constraint. > > > > gcc/testsuite/ChangeLog: > > > > * gcc.target/riscv/rvv/base/narrow_constraint-12.c: New test. > This is OK. > > The one question I keep having when I read these patterns is why we have > the earlyclobber. > > Earlyclobber means that the output is potentially written before the > inputs are consumed. Typically for a single instruction pattern such > constraints wouldn't make a lot of sense as *usually* the inputs are > consumed before the output is written. > > Just looking for a clarification as to why the earlyclobbers are needed > at all, particularly for non-reduction patterns. > > jeff >
