[llvm] Update default cutoff threshold for machine function splitter.

Based on internal testing at Google we found that setting the profile
summary cutoff threshold to 999950 yields the best results in terms of
itlb and icache metrics (as observed on Intel CPUs).

*default* = Split out code if no profile count available for block
*size-%*  = The fraction of bytes split out of .text and .text.hot
*itlb*    = Misses per kilo instructions (MPKI) for itlb
*icache*  = Misses per kilo instructions (MPKI) for L1 icache

Search1

| cutoff  | size-%  | itlb      | icache  |
|---------|---------|-----------|---------|
| default | 42.5861 | 0.0822151 | 2.46363 |
|  999999 | 44.9350 | 0.0767194 | 2.44416 |
|  999950 | 50.0660 |  0.075744 |  2.4091 |
|  999500 | 56.9158 |  0.082564 |  2.4188 |
|  995000 | 63.8625 | 0.0814927 | 2.42832 |
|  990000 | 71.7314 |  0.106906 | 2.57785 |

Search2

| cutoff  | size-% | itlb     | icache  |
|---------|--------|----------|---------|
| default | 2.8845 | 0.626712 | 4.73245 |
|  999999 | 3.3291 | 0.602309 | 4.70045 |
|  999950 | 3.8577 | 0.587842 | 4.71632 |
|  999500 | 4.4170 |  0.63577 | 4.68351 |
|  995000 | 5.1020 | 0.657969 | 4.82272 |
|  990000 | 5.7153 | 0.719122 | 5.39496 |

Differential Revision: https://reviews.llvm.org/D89085

diff --git a/llvm/lib/CodeGen/MachineFunctionSplitter.cpp b/llvm/lib/CodeGen/MachineFunctionSplitter.cpp
index 1a1bd90..b2f474a 100644 (file)
--- a/llvm/lib/CodeGen/MachineFunctionSplitter.cpp
+++ b/llvm/lib/CodeGen/MachineFunctionSplitter.cpp
@@ -39,11 +39,18 @@
 
 using namespace llvm;
 
+// FIXME: This cutoff value is CPU dependent and should be moved to
+// TargetTransformInfo once we consider enabling this on other platforms.
+// The value is expressed as a ProfileSummaryInfo integer percentile cutoff.
+// Defaults to 999950, i.e. all blocks colder than 99.995 percentile are split.
+// The default was empirically determined to be optimal when considering cutoff
+// values between 99%-ile to 100%-ile with respect to iTLB and icache metrics on
+// Intel CPUs.
 static cl::opt<unsigned>
     PercentileCutoff("mfs-psi-cutoff",
                      cl::desc("Percentile profile summary cutoff used to "
                               "determine cold blocks. Unused if set to zero."),
-                     cl::init(0), cl::Hidden);
+                     cl::init(999950), cl::Hidden);
 
 static cl::opt<unsigned> ColdCountThreshold(
     "mfs-count-threshold",

diff --git a/llvm/test/CodeGen/X86/machine-function-splitter.ll b/llvm/test/CodeGen/X86/machine-function-splitter.ll
index 188e471..5222b4d 100644 (file)
--- a/llvm/test/CodeGen/X86/machine-function-splitter.ll
+++ b/llvm/test/CodeGen/X86/machine-function-splitter.ll
@@ -1,5 +1,5 @@
 ; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -split-machine-functions | FileCheck %s -check-prefix=MFS-DEFAULTS
-; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -split-machine-functions -mfs-count-threshold=2000 | FileCheck %s --dump-input=always -check-prefix=MFS-OPTS1
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -split-machine-functions -mfs-psi-cutoff=0 -mfs-count-threshold=2000 | FileCheck %s --dump-input=always -check-prefix=MFS-OPTS1
 ; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -split-machine-functions -mfs-psi-cutoff=950000 | FileCheck %s -check-prefix=MFS-OPTS2
 
 define void @foo1(i1 zeroext %0) nounwind !prof !14 !section_prefix !15 {