Dominik Montada via llvm-dev
2020-Feb-14 13:49 UTC
[llvm-dev] [GlobalISel] How to properly handle the carry bit of G_UADDO/G_USUBO
Hi there, we are working on a backend in GlobalISel and have a question regarding the carry-in of the G_UADDO/G_USUBO instructions. Taking inspiration from the AMDGPU and X86 backend we introduced a pseudo register and non-allocatable register class which models our carry bit and use a COPY instruction to/from this pseudo register. For anything other than -O0 this already works fine, as the COPYs get eliminated. However on -O0 the COPY is not eliminated and we run into an assertion in our copyPhysReg implementation, because we cannot COPY to this pseudo register. Again looking at AMDGPU and X86 I found two possible solutions to this problem: AMDGPU seems to just emit an instruction which produces conditionally a carry bit. Since it's just -O0 anyway we don't care about these redundant instructions I guess. X86 on the other hand seems to be using a machine pass which eliminates COPYs to such a pseudo register. What would be the proper way to handle this? Also as a follow-up question: I noticed that I can mark the def of the COPY to the pseudo register as DEAD and it will get eliminated. However I am wondering if that is safe to do, even if its just a "pseudo copy" anyway. Can somebody give some feedback here? Cheers Dominik -- ---------------------------------------------------------------------- Dominik Montada Email: dominik.montada at hightec-rt.com HighTec EDV-Systeme GmbH Phone: +49 681 92613 19 Europaallee 19 Fax: +49-681-92613-26 D-66113 Saarbrücken WWW: http://www.hightec-rt.com Managing Director: Vera Strothmann Register Court: Saarbrücken, HRB 10445, VAT ID: DE 138344222 This e-mail may contain confidential and/or privileged information. If you are not the intended recipient please notify the sender immediately and destroy this e-mail. Any unauthorised copying, disclosure or distribution of the material in this e-mail is strictly forbidden. --- -------------- next part -------------- A non-text attachment was scrubbed... Name: smime.p7s Type: application/x-pkcs7-signature Size: 5409 bytes Desc: not available URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200214/a141f43a/attachment.bin>