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

[Vigneshwar Jayakumar via llvm-commits]

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@@ -409,6 +411,31 @@ class StructurizeCFGLegacyPass : public RegionPass {
 
 } // end anonymous namespace
 
+/// Helper function for heuristics to order if else block
+/// Checks whether an instruction is potential vector copy instruction, if so,
+/// checks if the operands are from different BB. if so, returns True.
+// Then there's a possibility of coelescing without interference when ordered
+// first.
+static bool hasAffectingInstructions(Instruction *I, BasicBlock *BB) {
+
+  if (!I || I->getParent() != BB)
+    return true;
+
+  // If the instruction is not a poterntial copy instructoin, return true.
+  if (!isa<ExtractElementInst>(*I) && !isa<ExtractValueInst>(*I))
+    return false;
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VigneshwarJ wrote:

The best way these heuristics will work is if we know which are going to be copy instructions after lowering. Here I just look at these two because mostly these are getting lowered to copy instruction. Another way of calculating heuristics for ordering was to see whether the incoming phi value instruction defined by the operands of the same block or different block. but this is too broad and can have unintentional reorders.

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