[PATCH] D159354: [SROA] Limit the number of allowed slices when trying to split allocas

[Dhruv Chawla via Phabricator via llvm-commits]

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This patch adds a hidden CLI option "--sroa-max-alloca-slices", which is
an integer that controls the maximum number of alloca slices SROA can
consider before bailing out. This prevents an issue with exponential compile
time explosion in passes like DSE and MemCpyOpt.

The default value has been chosen to prevent test breakages in the SROA
test suite while being low enough to prevent the unwanted behaviour.

Fixes https://github.com/rust-lang/rust/issues/88580.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D159354

Files:
  llvm/lib/Transforms/Scalar/SROA.cpp


Index: llvm/lib/Transforms/Scalar/SROA.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/SROA.cpp
+++ llvm/lib/Transforms/Scalar/SROA.cpp
@@ -121,6 +121,14 @@
 /// Disable running mem2reg during SROA in order to test or debug SROA.
 static cl::opt<bool> SROASkipMem2Reg("sroa-skip-mem2reg", cl::init(false),
                                      cl::Hidden);
+
+/// The maximum number of alloca slices allowed when splitting.
+static cl::opt<int>
+    SROAMaxAllocaSlices("sroa-max-alloca-slices", cl::init(32),
+                        cl::desc("Maximum number of alloca slices allowed "
+                                 "after which splitting is not attempted"),
+                        cl::Hidden);
+
 namespace {
 
 /// Calculate the fragment of a variable to use when slicing a store
@@ -4961,6 +4969,9 @@
   if (AS.isEscaped())
     return {Changed, CFGChanged};
 
+  if (std::distance(AS.begin(), AS.end()) > SROAMaxAllocaSlices)
+    return {Changed, CFGChanged};
+
   // Delete all the dead users of this alloca before splitting and rewriting it.
   for (Instruction *DeadUser : AS.getDeadUsers()) {
     // Free up everything used by this instruction.


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