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
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;
if (ViewBlockLayoutWithBFI != GVDT_None &&
(ViewBlockFreqFuncName.empty() ||
F->getFunction().getName().equals(ViewBlockFreqFuncName))) {
+ if (RenumberBlocksBeforeView)
+ MF.RenumberBlocks();
MBFI->view("MBP." + MF.getName(), false);
}