[PATCH] D29872: Do not legalize large add with addc/adde, introduce addcarry and do it with uaddo/addcarry

[James Y Knight via Phabricator via llvm-commits]

jyknight added a comment.

Haven't looked in great detail, but overall looks sane to me.

It'll be great to also eliminate ADDC/ADDE/SUBC/SUBE, instead of adding an alternative method. Oh...and, SETCCE. Will probably need a new variant of that as well?

The FIXME for x86 seems pretty distasteful...

Possibly could handle things by having ADDCARRY A, B, CARRY -> RES, OUTCARRY expand into something along the lines of:

1. Create FLAGS value per CARRY input. (what's best way to spell that? New pseudo-op "setcarry" that expands into "ADD8RI CARRY, 255 -> unused, FLAGS", maybe?)
2. Do addition, using carry frmo flags register, input and output: x86isd::ADC A, B, FLAGS -> RES, OUTFLAGS
3. Grab carry bit from flags register: x86isd::setcc cond_b, OUTFLAGS -> OUTCARRY

Then, have a transform that converts "(setcarry (setcc cond_b FLAGS)) -> FLAGS" into just passing through FLAGS?



================
Comment at: include/llvm/CodeGen/ISDOpcodes.h:232
+    /// nodes take three operands: The first two are the normal lhs and rhs to
+    /// the add or sub, and the third is a boolean coming from uaddo/usubo or
+    /// another addcarry/subcarry operation. The use of this opcode is preferable
----------------
Value should be allowed to come from anywhere, not just uaddo/addcarry, although those would be most common.


https://reviews.llvm.org/D29872