JIT: Run 3-opt once across all regions (dotnet#111989)

Part of dotnet#107749. 3-opt layout already refuses to align branches that cross EH regions, so there doesn't seem to be much utility in reordering each EH region independently. Thus, remove 3-opt's per-region ordering constraints, and run 3-opt once.

Author: amanasifkhalid

src/coreclr/jit/compiler.h

@@ -6326,7 +6326,6 @@ class Compiler
         BasicBlock** blockOrder;
         BasicBlock** tempOrder;
         unsigned numCandidateBlocks;
-        unsigned currEHRegion;
 
 #ifdef DEBUG
         weight_t GetLayoutCost(unsigned startPos, unsigned endPos);
@@ -6341,7 +6340,7 @@ class Compiler
         void AddNonFallthroughPreds(unsigned blockPos);
         bool RunGreedyThreeOptPass(unsigned startPos, unsigned endPos);
 
-        bool RunThreeOptPass(BasicBlock* startBlock, BasicBlock* endBlock);
+        bool RunThreeOptPass();
 
     public:
         ThreeOptLayout(Compiler* comp);

src/coreclr/jit/fgopt.cpp

@@ -4931,7 +4931,6 @@ Compiler::ThreeOptLayout::ThreeOptLayout(Compiler* comp)
     , blockOrder(nullptr)
     , tempOrder(nullptr)
     , numCandidateBlocks(0)
-    , currEHRegion(0)
 {
 }
 
@@ -5125,7 +5124,7 @@ void Compiler::ThreeOptLayout::ConsiderEdge(FlowEdge* edge)
     BasicBlock* const dstBlk = edge->getDestinationBlock();
 
     // Ignore cross-region branches
-    if ((srcBlk->bbTryIndex != currEHRegion) || (dstBlk->bbTryIndex != currEHRegion))
+    if (!BasicBlock::sameTryRegion(srcBlk, dstBlk))
     {
         return;
     }
@@ -5224,8 +5223,7 @@ void Compiler::ThreeOptLayout::AddNonFallthroughPreds(unsigned blockPos)
 }
 
 //-----------------------------------------------------------------------------
-// Compiler::ThreeOptLayout::Run: Runs 3-opt for each contiguous region of the block list
-// we're interested in reordering.
+// Compiler::ThreeOptLayout::Run: Runs 3-opt on the candidate span of hot blocks.
 // We skip reordering handler regions for now, as these are assumed to be cold.
 //
 void Compiler::ThreeOptLayout::Run()
@@ -5271,41 +5269,9 @@ void Compiler::ThreeOptLayout::Run()
 
         // Repurpose 'bbPostorderNum' for the block's ordinal
         block->bbPostorderNum = numCandidateBlocks++;
-
-        // While walking the span of blocks to reorder,
-        // remember where each try region ends within this span.
-        // We'll use this information to run 3-opt per region.
-        EHblkDsc* const HBtab = compiler->ehGetBlockTryDsc(block);
-        if (HBtab != nullptr)
-        {
-            HBtab->ebdTryLast = block;
-        }
-    }
-
-    // Reorder try regions first
-    bool modified = false;
-    for (EHblkDsc* const HBtab : EHClauses(compiler))
-    {
-        // If multiple region indices map to the same region,
-        // make sure we reorder its blocks only once
-        BasicBlock* const tryBeg = HBtab->ebdTryBeg;
-        if (tryBeg->getTryIndex() != currEHRegion++)
-        {
-            continue;
-        }
-
-        // Only reorder try regions within the candidate span of blocks
-        if ((tryBeg->bbPostorderNum < numCandidateBlocks) && (blockOrder[tryBeg->bbPostorderNum] == tryBeg))
-        {
-            JITDUMP("Running 3-opt for try region #%d\n", (currEHRegion - 1));
-            modified |= RunThreeOptPass(tryBeg, HBtab->ebdTryLast);
-        }
     }
 
-    // Finally, reorder the main method body
-    currEHRegion = 0;
-    JITDUMP("Running 3-opt for main method body\n");
-    modified |= RunThreeOptPass(compiler->fgFirstBB, blockOrder[numCandidateBlocks - 1]);
+    const bool modified = RunThreeOptPass();
 
     if (modified)
     {
@@ -5314,14 +5280,25 @@ void Compiler::ThreeOptLayout::Run()
             BasicBlock* const block = blockOrder[i - 1];
             BasicBlock* const next  = blockOrder[i];
 
+            if (block->NextIs(next))
+            {
+                continue;
+            }
+
             // Only reorder within EH regions to maintain contiguity.
-            // TODO: Allow moving blocks in different regions when 'next' is the region entry.
-            // This would allow us to move entire regions up/down because of the contiguity requirement.
-            if (!block->NextIs(next) && BasicBlock::sameEHRegion(block, next))
+            if (!BasicBlock::sameEHRegion(block, next))
+            {
+                continue;
+            }
+
+            // Don't move the entry of an EH region.
+            if (compiler->bbIsTryBeg(next) || compiler->bbIsHandlerBeg(next))
             {
-                compiler->fgUnlinkBlock(next);
-                compiler->fgInsertBBafter(block, next);
+                continue;
             }
+
+            compiler->fgUnlinkBlock(next);
+            compiler->fgInsertBBafter(block, next);
         }
     }
 }
@@ -5466,12 +5443,6 @@ bool Compiler::ThreeOptLayout::RunGreedyThreeOptPass(unsigned startPos, unsigned
                     continue;
                 }
 
-                // Don't consider any cut points that would disturb other EH regions
-                if (!BasicBlock::sameEHRegion(s2Block, s3Block))
-                {
-                    continue;
-                }
-
                 // Compute the cost delta of this partition
                 const weight_t currCost = currCostBase + GetCost(s3BlockPrev, s3Block);
                 const weight_t newCost =
@@ -5529,22 +5500,15 @@ bool Compiler::ThreeOptLayout::RunGreedyThreeOptPass(unsigned startPos, unsigned
 }
 
 //-----------------------------------------------------------------------------
-// Compiler::ThreeOptLayout::RunThreeOptPass: Runs 3-opt for the given block range.
-//
-// Parameters:
-//   startBlock - The first block of the range to reorder
-//   endBlock - The last block (inclusive) of the range to reorder
+// Compiler::ThreeOptLayout::RunThreeOptPass: Runs 3-opt on the candidate span of blocks.
 //
 // Returns:
 //   True if we reordered anything, false otherwise
 //
-bool Compiler::ThreeOptLayout::RunThreeOptPass(BasicBlock* startBlock, BasicBlock* endBlock)
+bool Compiler::ThreeOptLayout::RunThreeOptPass()
 {
-    assert(startBlock != nullptr);
-    assert(endBlock != nullptr);
-
-    const unsigned startPos  = startBlock->bbPostorderNum;
-    const unsigned endPos    = endBlock->bbPostorderNum;
+    const unsigned startPos  = 0;
+    const unsigned endPos    = numCandidateBlocks - 1;
     const unsigned numBlocks = (endPos - startPos + 1);
     assert(startPos <= endPos);