[PATCH] D107658: [RISCV] Teach isel to select ADDW/SUBW/MULW/SLLIW when only the lower 32-bits are used.

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

In D107658#2951066 <https://reviews.llvm.org/D107658#2951066>, @jrtc27 wrote:

> What's the reasoning behind the current set of opcodes? E.g. are there cases where div[u]w/rem[u]w or sra[i]w/srl[i]w are worth using?

These are the only opcodes that are type legalized by any_extend because the result in the lower 32 bits doesn't depend on the upper 32 bits after promotion. The other binary operators use RISCVISD::*W ISD opcodes or require a sign_extend_inreg/zero_extend_inreg because the upper 32 bits can effect the lower 32 bits. These are also the only opcodes used in an isel pattern that starts with sext_inreg.

I'm probably going to explain the rest of this poorly, but I'll try.

div[u]w/remuw are converted to their own special ISD opcode during type legalization. I believe there was a subtle issue with pattern matching ISD::DIV/UDIV/UREM and zext_inreg/sext_inreg to the W instructions. We can only pattern match ISD::SREM+sext_inreg to REMW.

sraiw/srliw are type legalized by inserting a sext_inreg or zext_inreg. We select SRAIW/SRLIW based on the inputs being a sext_inreg/zext_inreg(or the equivalent computeKnownBits/computeNumSignBits) and the immediate fitting in 5 bits. We can't select SRAIW/SRLIW based on how the result is used. We need to know that sign bits or 0 are supposed to be shifted into bit 31.

sraw/srlw are type legalized using their own RISCVISD::SRAW/SRLW nodes. Since we can't see the shift amount, we need these ISD opcodes in order to know that it was UB for the shift amount to be more than 5 bits. Default type legalization would insert a sext_inreg/zext_inreg, but we can't pattern match that if we don't know sure that it came from a type legalized i32 shift and an i64 shift that happened to have a sext_inreg/zext_inreg input. If it came from an i64 shift we might incorrectly ignore bit 5 of the shift amount by turning it into a W instruction.


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