[NFC][BlockPlacement]Add an option to renumber blocks based on function layout order.

[mingmingl]

Summary

Use case:

• When block layout is visualized after MBP pass, the basic blocks are labeled in layout order; meanwhile blocks could be numbered in a different order.
• As a result, it's hard to map between the graph and pass output. With this option on, the basic blocks are renumbered in function layout order.

This option is only useful when a function is to be visualized (i.e., when view options are on) to make it debugging only.

Use https://godbolt.org/z/5WTW36bMr as an example:

• As MBP pass output (shown in godbolt output window), func2 is in a basic block numbered 2 (bb.2), and func1 is in a basic block numbered 3 (bb.3); bb.3 is a block with higher block frequency than bb.2, and bb.3 is placed before bb.2 in the functin layout.
• Use [1] to get the dot graph (graph uploaded in [2]), the blocks are re-numbered.
  • func1 is in 'if.end' block, and labeled 1 in visualized dot; func2 is in 'if.then' blocks, and labeled 3 --> the labeled number and bb number won't map.
  • DOTGraphTraits<MachineBlockFrequencyInfo *>::getNodeLabel is where labeled numbers are based on function layout number, and called by graph writer. So call 'MachineFunction::RenumberBlocks' would make labeled number (in dot graph) and block number (in pass output) consistent with each other.

[1] ./bin/clang++ -O3 -S -mllvm -view-block-layout-with-bfi=count -mllvm -view-bfi-func-name=_Z9func_loopv -mllvm -print-after=block-placement -mllvm -filter-print-funcs=_Z9func_loopv test.c

[2]

[davidxl]

Does MIR text output show MBB number in the dump?

[davidxl]

or show an example how the mapping is made harder without renumbering.

[mingmingl]

In D137467#3909358, @davidxl wrote:

Does MIR text output show MBB number in the dump?

Yes, the basic blocks are named in format bb.<N>. Added an example in the summary.

In D137467#3909359, @davidxl wrote:

or show an example how the mapping is made harder without renumbering.

Added an example in Phabricator summary.

[davidxl]

I see. The DOT CFG graph dump uses the layout order instead of the MBB number. Another way to fix this is to use the following in the node label: BBName[MBBNum][layout-order], but making the MBB num and layout number consistent under an option is fine to me.

[davidxl]

lgtm

[mingmingl]

In D137467#3911238, @davidxl wrote:

I see. The DOT CFG graph dump uses the layout order instead of the MBB number. Another way to fix this is to use the following in the node label: BBName[MBBNum][layout-order], but making the MBB num and layout number consistent under an option is fine to me.

Using 'BBName[MBBNum][layout-order]' would keep more information; but it requires more implementation work.

Going to submit this. Thanks for reviews!

llvm/lib/CodeGen/MachineBlockPlacement.cpp

 // Heuristic for triangle chains.
 static cl::opt<unsigned> TriangleChainCount(
     "triangle-chain-count",
     cl::desc("Number of triangle-shaped-CFG's that need to be in a row for the "
              "triangle tail duplication heuristic to kick in. 0 to disable."),
     cl::init(2),
     cl::Hidden);
 
+// Use case: When block layout is visualized after MBP pass, the basic blocks
+// are labeled in layout order; meanwhile blocks could be numbered in a
+// different order. It's hard to map between the graph and pass output.
+// With this option on, the basic blocks are renumbered in function layout
+// order. For debugging only.
+static cl::opt<bool> RenumberBlocksBeforeView(
+    "renumber-blocks-before-view",
+    cl::desc(
+        "If true, basic blocks are re-numbered before MBP layout is printed "
+        "into a dot graph. Only used when a function is being printed."),
+    cl::init(false), cl::Hidden);
+
 extern cl::opt<bool> EnableExtTspBlockPlacement;
 extern cl::opt<bool> ApplyExtTspWithoutProfile;
 
 namespace llvm {
 extern cl::opt<unsigned> StaticLikelyProb;
 extern cl::opt<unsigned> ProfileLikelyProb;
 
 // Internal option used to control BFI display only after MBP pass.
for (auto MBI = std::next(MF.begin()), MBE = MF.end(); MBI != MBE; ++MBI) {
         else
           MBI->setAlignment(Align(1ULL << AlignAllNonFallThruBlocks));
       }
     }
   }
   if (ViewBlockLayoutWithBFI != GVDT_None &&
       (ViewBlockFreqFuncName.empty() ||
        F->getFunction().getName().equals(ViewBlockFreqFuncName))) {
+    if (RenumberBlocksBeforeView)
+      MF.RenumberBlocks();
     MBFI->view("MBP." + MF.getName(), false);
   }
 
   // We always return true as we have no way to track whether the final order
   // differs from the original order.
   return true;
 }