[Pipelines] Additional unrolling in LTO

OmaxLTO.cfg

@@ -1,3 +1,4 @@
 -flto=full \
 -fvirtual-function-elimination \
--fwhole-program-vtables
+-fwhole-program-vtables \
+-mllvm -extra-LTO-loop-unroll=true

README.md

@@ -172,6 +172,7 @@ and/or increased memory usage during linking. Some of the options in the config
 corresponding optimisation passes in the [LLVM project](https://github.com/llvm/llvm-project)
 to find out more. Users are also encouraged to create their own configs and tune their own
 flag parameters.
+Information on LLVM Embedded Toolchain for Arm specific optimization flags is available in [Optimization Flags](https://github.com/ARM-software/LLVM-embedded-toolchain-for-Arm/blob/main/docs/optimization-flags.md)
 
 Binary releases of the LLVM Embedded Toolchain for Arm are based on release
 branches of the upstream LLVM Project, thus can safely be used with all tools

docs/optimization-flags.md

@@ -0,0 +1,9 @@
+Additional optimization flags
+=============================
+
+## Additional loop unroll in the LTO pipeline
+In some cases it is benefitial to perform an additional loop unroll pass so that extra information becomes available to later passes, e.g. SROA. 
+Use cases where this could be beneficial - multiple (N>=4) nested loops.
+
+### Usage: 
+    -mllvm -extra-LTO-loop-unroll=true/false

patches/llvm-project-perf/0000-LTOpasses-add-loop-unroll.patch

@@ -0,0 +1,55 @@
+From 4adfc5231d2c0182d6278b4aa75eec57648e5dd4 Mon Sep 17 00:00:00 2001
+From: Vladi Krapp <[email protected]>
+Date: Tue, 3 Sep 2024 14:12:48 +0100
+Subject: [Pipelines] Additional unrolling  in LTO 
+
+Some workloads require specific sequences of events to happen
+to fully simplify. This adds an extra full unrolling pass to help these
+cases on the cores with branch predictors. It helps produce simplified
+loops, which can then be SROA'd allowing further simplification, which
+can be important for performance.
+This is added under own flag - spending extra compile time to get extra
+performance on specific user request.
+
+Originally patch by David Green ([email protected])
+---
+ llvm/lib/Passes/PassBuilderPipelines.cpp | 16 ++++++++++++++++
+ 1 file changed, 16 insertions(+)
+
+diff --git a/llvm/lib/Passes/PassBuilderPipelines.cpp b/llvm/lib/Passes/PassBuilderPipelines.cpp
+index 1184123c7710..6dc45d85927a 100644
+--- a/llvm/lib/Passes/PassBuilderPipelines.cpp
++++ b/llvm/lib/Passes/PassBuilderPipelines.cpp
+@@ -332,6 +332,10 @@ namespace llvm {
+ extern cl::opt<unsigned> MaxDevirtIterations;
+ } // namespace llvm
+
++static cl::opt<bool> LTOExtraLoopUnroll(
++    "extra-LTO-loop-unroll", cl::init(false), cl::Hidden,
++    cl::desc("Perform extra loop unrolling pass to assist SROA"));
++
+ void PassBuilder::invokePeepholeEPCallbacks(FunctionPassManager &FPM,
+                                             OptimizationLevel Level) {
+   for (auto &C : PeepholeEPCallbacks)
+@@ -1940,6 +1944,18 @@ PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
+   MPM.addPass(createModuleToPostOrderCGSCCPassAdaptor(ArgumentPromotionPass()));
+
+   FunctionPassManager FPM;
++
++  if (LTOExtraLoopUnroll) {
++    LoopPassManager OmaxLPM;
++    OmaxLPM.addPass(LoopFullUnrollPass(Level.getSpeedupLevel(),
++                                       /* OnlyWhenForced= */ !PTO.LoopUnrolling,
++                                       PTO.ForgetAllSCEVInLoopUnroll));
++    FPM.addPass(
++        createFunctionToLoopPassAdaptor(std::move(OmaxLPM),
++                                        /*UseMemorySSA=*/false,
++                                        /*UseBlockFrequencyInfo=*/true));
++  }
++
+   // The IPO Passes may leave cruft around. Clean up after them.
+   FPM.addPass(InstCombinePass());
+   invokePeepholeEPCallbacks(FPM, Level);
+-- 
+2.34.1
+