[PATCH] D109553: [SCEV] Attempt to define what flags are legal on a SCEV
Philip Reames via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Thu Sep 9 15:54:27 PDT 2021
reames created this revision.
reames added reviewers: efriedma, nikic, mkazantsev, skatkov, lebedev.ri, fhahn.
Herald added subscribers: bollu, mcrosier.
reames requested review of this revision.
Herald added a project: LLVM.
This is a comment only change, inspired by the discussion on D106852 <https://reviews.llvm.org/D106852>.
This is my attempt to define what the current semantics are closest too. The existence of D106852 <https://reviews.llvm.org/D106852> does imply these semantics are not 100% correct, but that piece of code is the only place I can find in current code which violates them.
If we accept these semantics, then the resolution to D106852 <https://reviews.llvm.org/D106852> becomes obvious, if painful.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D109553
Files:
llvm/include/llvm/Analysis/ScalarEvolution.h
Index: llvm/include/llvm/Analysis/ScalarEvolution.h
===================================================================
--- llvm/include/llvm/Analysis/ScalarEvolution.h
+++ llvm/include/llvm/Analysis/ScalarEvolution.h
@@ -112,6 +112,25 @@
/// Note that NUW and NSW are also valid properties of a recurrence, and
/// either implies NW. For convenience, NW will be set for a recurrence
/// whenever either NUW or NSW are set.
+ ///
+ /// We require that the flag on a SCEV apply to the entire scope in which
+ /// that SCEV is defined. A SCEV's scope is set of locations dominated by
+ /// a defining location, which is in turn describes by the following rules:
+ /// * A SCEVUnknown is at the point of definition of the Value.
+ /// * A SCEVConstant is defined at all points.
+ /// * A SCEVAddRec is defined no earlier than the header of the associated
+ /// loop. If any operand is loop varying (per these definition rules),
+ /// than the SCEVAddRec is defined only after all operands are defined.
+ /// * All other SCEVs are defined at the earlest point all operands are
+ /// defined.
+ ///
+ /// The above rules describe a maximally hoisted form (without regards to
+ /// potential control dependence). A SCEV is defined anywhere a
+ /// corresponding instruction could be defined in said maximally hoisted
+ /// form. Note that SCEVUDivExpr (currently the only expression type which
+ /// can trap) can be defined per these rules in regions where it would trap
+ /// at runtime. A SCEV being defined does not require the existance of any
+ /// instruction within the defined scope.
enum NoWrapFlags {
FlagAnyWrap = 0, // No guarantee.
FlagNW = (1 << 0), // No self-wrap.
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