[flang-commits] [flang] e215324 - [flang][StackArrays] skip analysis of very large functions (#71047)

[via flang-commits]

Author: Tom Eccles
Date: 2023-11-03T10:29:33Z
New Revision: e21532418528f0cbcec4c811ce0ea41e8b3075fe

URL: https://github.com/llvm/llvm-project/commit/e21532418528f0cbcec4c811ce0ea41e8b3075fe
DIFF: https://github.com/llvm/llvm-project/commit/e21532418528f0cbcec4c811ce0ea41e8b3075fe.diff

LOG: [flang][StackArrays] skip analysis of very large functions (#71047)

The stack arrays pass uses data flow analysis to determine whether heap
allocations are freed on all paths out of the function.

`interp_domain_em_part2` in spec2017 wrf generates over 120k operations,
including almost 5k fir.if operations and over 200 fir.do_loop
operations, all in the same function. The MLIR data flow analysis
framework cannot provide reasonable performance for such cases because
there is a combinatorial explosion in the number of control flow paths
through the function, all of which must be checked to determine if the
heap allocations will be freed.

This patch skips the stack arrays pass for ridiculously large functions
(defined as having more than 1000 fir.allocmem operations). This
threshold is configurable at runtime with a command line argument.

With this patch, compiling this file is more than 80% faster.

Added: 
    

Modified: 
    flang/lib/Optimizer/Transforms/StackArrays.cpp

Removed: 
    


################################################################################
diff  --git a/flang/lib/Optimizer/Transforms/StackArrays.cpp b/flang/lib/Optimizer/Transforms/StackArrays.cpp
index 9b90aed5a17ae73..b51e2aae1a9d510 100644
--- a/flang/lib/Optimizer/Transforms/StackArrays.cpp
+++ b/flang/lib/Optimizer/Transforms/StackArrays.cpp
@@ -42,6 +42,13 @@ namespace fir {
 
 #define DEBUG_TYPE "stack-arrays"
 
+static llvm::cl::opt<std::size_t> maxAllocsPerFunc(
+    "stack-arrays-max-allocs",
+    llvm::cl::desc("The maximum number of heap allocations to consider in one "
+                   "function before skipping (to save compilation time). Set "
+                   "to 0 for no limit."),
+    llvm::cl::init(1000), llvm::cl::Hidden);
+
 namespace {
 
 /// The state of an SSA value at each program point
@@ -411,6 +418,17 @@ void AllocationAnalysis::processOperation(mlir::Operation *op) {
 mlir::LogicalResult
 StackArraysAnalysisWrapper::analyseFunction(mlir::Operation *func) {
   assert(mlir::isa<mlir::func::FuncOp>(func));
+  size_t nAllocs = 0;
+  func->walk([&nAllocs](fir::AllocMemOp) { nAllocs++; });
+  // don't bother with the analysis if there are no heap allocations
+  if (nAllocs == 0)
+    return mlir::success();
+  if ((maxAllocsPerFunc != 0) && (nAllocs > maxAllocsPerFunc)) {
+    LLVM_DEBUG(llvm::dbgs() << "Skipping stack arrays for function with "
+                            << nAllocs << " heap allocations");
+    return mlir::success();
+  }
+
   mlir::DataFlowSolver solver;
   // constant propagation is required for dead code analysis, dead code analysis
   // is required to mark blocks live (required for mlir dense dfa)