[llvm] [StructurizeCFG] Order IF Else block using Heuristics (PR #139605)

[Matt Arsenault via llvm-commits]

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@@ -419,6 +446,58 @@ INITIALIZE_PASS_DEPENDENCY(RegionInfoPass)
 INITIALIZE_PASS_END(StructurizeCFGLegacyPass, "structurizecfg",
                     "Structurize the CFG", false, false)
 
+/// Then and Else block order in SCC is arbitrary. But based on the
+/// order, after structurization there are cases where there might be extra
+/// VGPR copies due to interference during register coelescing.
+///  eg:- incoming phi values from Else block contains only vgpr copies and
+///  incoming phis in Then block has are some modification for the vgprs.
+/// after structurization, there would be interference when coelesing when Then
+/// block is ordered first. But those copies can be coelesced when Else is
+/// ordered first.
+///
+/// This function checks the incoming phi values in the merge block and
+/// orders based on the following heuristics  of Then and Else block. Checks
+/// whether an incoming phi can be potential copy instructions and if so
+/// checks whether copy within the block or not. 
+/// Increases score if its a potential copy from outside the block. 
+/// the higher scored block is ordered first.
+void StructurizeCFG::reorderIfElseBlock(BasicBlock *BB, unsigned Idx) {
+  BranchInst *Term = dyn_cast<BranchInst>(BB->getTerminator());
+
+  if (Term && Term->isConditional()) {
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arsenm wrote:

Prefer early return to nested conditionals 

https://github.com/llvm/llvm-project/pull/139605