[llvm] [SLSR] Adding a cost model considering register pressure (PR #213808)

[Igor Wodiany via llvm-commits]

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@@ -1378,11 +1505,50 @@ bool StraightLineStrengthReduce::runOnFunction(Function &F) {
   }
   sortCandidateInstructions();
 
+  // From SortedCandidateInsts, remove some candidates that are likely to
+  // increase register pressure. The candidate's Inst is the source of
+  // replacement. A candidate in the following criteria should be removed:
+  // 1. The candidate's Inst "has operands used in non-rewritable users in
+  // another block"
+  //    -- checked by hasOperandsUsedInNonRewritableUsersInAnotherBlock(Inst)
+  //    -- This means the candidate's Inst's original operands are live in
+  //    another block, so even if rewrite the Inst, the operands may be still
+  //    live out to another block.
+  //    -- Thus, rewriting the Inst based on Basis might add another long live
+  //    range from the Basis by increasing the live range of the Basis.
+  //    -- TODO: If needed, a refinement to check that "another block" is
+  //    properly dominated by the candidate's Inst's block can be added.
+  // 2. When the candidate's Basis's is only used in the the same block
+  // and its last use is before the candidate's Inst, the difference between the
+  // last use of Basis and the Inst is larger than a threshold.
+  //    -- This is also for avoiding increasing the live range of the Basis by
+  //    rewriting the Inst.
+  //
+  // A candidate satisfies both conditions 1 and 2 should be removed.
+
+  // Collect candidates likely to increase register pressure.
+  // Evaluate on the original IR, before any rewriteCandidate mutates it
+  // Done before rewriting: rewriting inserts instructions and does
+  // replaceAllUsesWith, which would invalidate both the in-block index map and
+  // operands' user sets.
+  DenseSet<Instruction *> ToSkipRewrite;
+  {
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IgWod wrote:

Any reason to have braces around this code?

https://github.com/llvm/llvm-project/pull/213808