On Mon, 2020-01-27 at 16:41 +0000, Richard Sandiford wrote: > In the gcc.target/aarch64/lsl_asr_sbfiz.c part of this PR, we have: > > Failed to match this instruction: > (set (reg:SI 95) > (ashift:SI (subreg:SI (sign_extract:DI (subreg:DI (reg:SI 97) 0) > (const_int 3 [0x3]) > (const_int 0 [0])) 0) > (const_int 19 [0x13]))) > > If we perform the natural simplification to: > > (set (reg:SI 95) > (ashift:SI (sign_extract:SI (reg:SI 97) > (const_int 3 [0x3]) > (const_int 0 [0])) 0) > (const_int 19 [0x13]))) > > then the pattern matches. And it turns out that we do have a > simplification like that already, but it would only kick in for > extractions from a reg, not a subreg. E.g.: Yea. I ran into similar problems with the extract/extend bits in combine. And we know it's a fairly general problem that we don't handle SUBREGs anywhere near as consistently as REGs.
> > (set (reg:SI 95) > (ashift:SI (subreg:SI (sign_extract:DI (reg:DI X) > (const_int 3 [0x3]) > (const_int 0 [0])) 0) > (const_int 19 [0x13]))) > > would simplify to: > > (set (reg:SI 95) > (ashift:SI (sign_extract:SI (subreg:SI (reg:DI X) 0) > (const_int 3 [0x3]) > (const_int 0 [0])) 0) > (const_int 19 [0x13]))) > > IMO the subreg case is even more obviously a simplification > than the bare reg case, since the net effect is to remove > either one or two subregs, rather than simply change the > position of a subreg/truncation. > > However, doing that regressed gcc.dg/tree-ssa/pr64910-2.c > for -m32 on x86_64-linux-gnu, because we could then simplify > a :HI zero_extract to a :QI one. The associated *testqi_ext_3 > pattern did already seem to want to handle QImode extractions: > > "ix86_match_ccmode (insn, CCNOmode) > && ((TARGET_64BIT && GET_MODE (operands[2]) == DImode) > || GET_MODE (operands[2]) == SImode > || GET_MODE (operands[2]) == HImode > || GET_MODE (operands[2]) == QImode) > > but I'm not sure how often the QI case would trigger in practice, > since the zero_extract mode was restricted to HI and above. I checked > the other x86 patterns and couldn't see any other instances of this. > > Tested on aarch64-linux-gnu, x86_64-linux-gnu and powerpc64le-linux-gnu, > OK to install? > > Richard > > > 2020-01-27 Richard Sandiford <richard.sandif...@arm.com> > > gcc/ > PR rtl-optimization/87763 > * simplify-rtx.c (simplify_truncation): Extend sign/zero_extract > simplification to handle subregs as well as bare regs. > * config/i386/i386.md (*testqi_ext_3): Match QI extracts too. Do you need to check for and reject paradoxicals here? If not, OK as- is. If you need to check, then that's pre-approved as well. jeff >
