[PATCH] D125130: [CodeGen] Clarify the semantics of ADDCARRY/SUBCARRY
Kazu Hirata via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Sat May 7 13:15:54 PDT 2022
kazu updated this revision to Diff 427892.
kazu added a comment.
Included a comparison to [US]{ADD,SUB}O.
Split the description to several paragraphs, separating the definition
proper and supplemental information.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D125130/new/
https://reviews.llvm.org/D125130
Files:
llvm/include/llvm/CodeGen/ISDOpcodes.h
Index: llvm/include/llvm/CodeGen/ISDOpcodes.h
===================================================================
--- llvm/include/llvm/CodeGen/ISDOpcodes.h
+++ llvm/include/llvm/CodeGen/ISDOpcodes.h
@@ -281,12 +281,25 @@
/// Carry-using nodes for multiple precision addition and subtraction.
/// These nodes take three operands: The first two are the normal lhs and
- /// rhs to the add or sub, and the third is a boolean indicating if there
- /// is an incoming carry. These nodes produce two results: the normal
- /// result of the add or sub, and the output carry so they can be chained
- /// together. The use of this opcode is preferable to adde/sube if the
- /// target supports it, as the carry is a regular value rather than a
- /// glue, which allows further optimisation.
+ /// rhs to the add or sub, and the third is a boolean value that is 1 if and
+ /// only if there is an incoming carry/borrow. These nodes produce two
+ /// results: the normal result of the add or sub, and a boolean value that is
+ /// 1 if and only if there is an outgoing carry/borrow.
+ ///
+ /// Care must be taken if these opcodes are lowered to hardware instructions
+ /// that use the inverse logic -- 0 if and only if there is an
+ /// incoming/outgoing carry/borrow. In such cases, you must preserve the
+ /// semantcis of these opcodes by inverting the incoming carry/borrow, feeding
+ /// it to the sub/add hardware instruction, and then inverting the outgoing
+ /// carry/borrow.
+ ///
+ /// The use of these opcodes is preferable to adde/sube if the target supports
+ /// it, as the carry is a regular value rather than a glue, which allows
+ /// further optimisation.
+ ///
+ /// These opcodes are different from [US]{ADD,SUB}O in that ADDCARRY/SUBCARRY
+ /// consume and produce a carry/borrow, whereas [US]{ADD,SUB}O produce an
+ /// overflow.
ADDCARRY,
SUBCARRY,
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