[llvm] 36e8e19 - [NFC][BlockPlacement]Add an option to renumber blocks based on function layout order.

[Mingming Liu via llvm-commits]

Author: Mingming Liu
Date: 2022-11-07T07:52:45-08:00
New Revision: 36e8e19337b9a6f47c7f17c849164df47d45fa65

URL: https://github.com/llvm/llvm-project/commit/36e8e19337b9a6f47c7f17c849164df47d45fa65
DIFF: https://github.com/llvm/llvm-project/commit/36e8e19337b9a6f47c7f17c849164df47d45fa65.diff

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

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.
   - [[ https://github.com/llvm/llvm-project/blob/b5626ae9751f0d82aa04791a21689b289721738e/llvm/lib/CodeGen/MachineBlockFrequencyInfo.cpp#L127 | DOTGraphTraits<MachineBlockFrequencyInfo *>::getNodeLabel ]] is where labeled numbers are based on function layout number, and [[ https://github.com/llvm/llvm-project/blob/a8d93783f37c042ace67069ae4ca6f8fd849c2d0/llvm/include/llvm/Support/GraphWriter.h#L209
 | 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] {F25201785}

Reviewed By: davidxl

Differential Revision: https://reviews.llvm.org/D137467

Added: 
    

Modified: 
    llvm/lib/CodeGen/MachineBlockPlacement.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index 4884ac9417204..7bbc347a8cf88 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -201,6 +201,18 @@ static cl::opt<unsigned> TriangleChainCount(
     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
+// 
diff erent 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;
 
@@ -3466,6 +3478,8 @@ bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) {
   if (ViewBlockLayoutWithBFI != GVDT_None &&
       (ViewBlockFreqFuncName.empty() ||
        F->getFunction().getName().equals(ViewBlockFreqFuncName))) {
+    if (RenumberBlocksBeforeView)
+      MF.RenumberBlocks();
     MBFI->view("MBP." + MF.getName(), false);
   }