[llvm] 24bf6ff - [llvm] Update default cutoff threshold for machine function splitter.
Snehasish Kumar via llvm-commits
llvm-commits at lists.llvm.org
Wed Oct 14 12:51:05 PDT 2020
Author: Snehasish Kumar
Date: 2020-10-14T12:48:10-07:00
New Revision: 24bf6ff4e08f88df0b6c01ef87aa384276636901
URL: https://github.com/llvm/llvm-project/commit/24bf6ff4e08f88df0b6c01ef87aa384276636901
DIFF: https://github.com/llvm/llvm-project/commit/24bf6ff4e08f88df0b6c01ef87aa384276636901.diff
LOG: [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
Added:
Modified:
llvm/lib/CodeGen/MachineFunctionSplitter.cpp
llvm/test/CodeGen/X86/machine-function-splitter.ll
Removed:
################################################################################
diff --git a/llvm/lib/CodeGen/MachineFunctionSplitter.cpp b/llvm/lib/CodeGen/MachineFunctionSplitter.cpp
index 1a1bd901c92d..b2f474afea99 100644
--- 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 188e4716349d..5222b4d96cf1 100644
--- 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 {
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