https://llvm.org/bugs/show_bug.cgi?id=24618
Bug ID: 24618
Summary: Compilation wtih Clang is 8x slower than GCC, mostly
due to Greedy Register Allocator
Product: libraries
Version: trunk
Hardware: HP
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Register Allocator
Assignee: unassignedb...@nondot.org
Reporter: cmt...@google.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
I have a .cpp file that takes 2 minutes and 45 seconds to compile with clang
(ToT), and only 25 seconds to compile with gcc. Both at -O3 (times were
obtained with the unix 'time' command):
Time compiling with clang:
real 2m45.683s
user 2m45.023s
sys 0m0.539s
Time compiling with clang:
real 0m23.502s
user 0m21.340s
sys 0m2.134s
I passed -ftime-report to clang, to find out where the time was being spent and
it reported that 75% of the time is being spent int he Greedy Register
Allocator:
[...snip...]
===-------------------------------------------------------------------------===
Register Allocation
===-------------------------------------------------------------------------===
Total Execution Time: 106.5713 seconds (106.6387 wall clock)
---User Time--- --System Time-- --User+System-- ---Wall Time--- ---
Name ---
104.8476 ( 98.4%) 0.0202 ( 50.4%) 104.8678 ( 98.4%) 104.9201 ( 98.4%)
Spiller
1.6598 ( 1.6%) 0.0080 ( 19.9%) 1.6678 ( 1.6%) 1.6695 ( 1.6%)
Global Splitting
0.0210 ( 0.0%) 0.0119 ( 29.7%) 0.0329 ( 0.0%) 0.0462 ( 0.0%) Evict
0.0018 ( 0.0%) 0.0000 ( 0.0%) 0.0018 ( 0.0%) 0.0021 ( 0.0%) Seed
Live Regs
0.0010 ( 0.0%) 0.0000 ( 0.0%) 0.0010 ( 0.0%) 0.0009 ( 0.0%) Local
Splitting
106.5312 (100.0%) 0.0400 (100.0%) 106.5713 (100.0%) 106.6387 (100.0%)
Total
[...snip...]
===-------------------------------------------------------------------------===
... Pass execution timing report ...
===-------------------------------------------------------------------------===
Total Execution Time: 142.7354 seconds (142.7899 wall clock)
---User Time--- --System Time-- --User+System-- ---Wall Time--- ---
Name ---
107.0321 ( 75.4%) 0.0559 ( 6.4%) 107.0881 ( 75.0%) 107.1572 ( 75.0%)
Greedy Register Allocator
5.3523 ( 3.8%) 0.0799 ( 9.1%) 5.4321 ( 3.8%) 5.4367 ( 3.8%) Tail
Duplication
5.2889 ( 3.7%) 0.0000 ( 0.0%) 5.2889 ( 3.7%) 5.2914 ( 3.7%)
Eliminate PHI nodes for register allocation
3.6523 ( 2.6%) 0.0040 ( 0.5%) 3.6562 ( 2.6%) 3.6589 ( 2.6%)
Simple Register Coalescing
I will try to attach the entire timing report if I can attach two files to this
bug. I also ran the compilation (on clang) under 'perf' to try to see where
time was being spent, and again, it looks like a large chuck of time is being
spend in the register allocator:
+ 15.89% clang++ clang-3.8 [.] (anonymous
namespace)::InlineSpiller::propagateSiblingValue(llvm::DenseMapIterator<llvm::VNInfo*,
(anonymous namespace)::InlineSpiller::SibValueInfo, llvm::DenseMapInfo<▒
+ 11.19% clang++ clang-3.8 [.]
llvm::MachineBlockFrequencyInfo::getBlockFreq(llvm::MachineBasicBlock const*)
const
▒
+ 5.33% clang++ clang-3.8 [.]
llvm::BlockFrequencyInfoImplBase::getBlockFreq(llvm::BlockFrequencyInfoImplBase::BlockNode
const&) const ▒
+ 3.79% clang++ clang-3.8 [.]
llvm::SmallPtrSetImplBase::FindBucketFor(void const*) const
▒
+ 3.21% clang++ clang-3.8 [.]
llvm::SSAUpdaterImpl<llvm::MachineSSAUpdater>::BuildBlockList(llvm::MachineBasicBlock*,
llvm::SmallVectorImpl<llvm::SSAUpdaterImpl<llvm::MachineSSAUpdater>::BBInfo*>*)
▒
+ 2.84% clang++ clang-3.8 [.]
llvm::LiveVariables::isLiveOut(unsigned int, llvm::MachineBasicBlock const&)
▒
+ 2.68% clang++ clang-3.8 [.]
llvm::LiveRange::addSegment(llvm::LiveRange::Segment)
▒
+ 2.62% clang++ clang-3.8 [.]
llvm::SmallPtrSetImplBase::insert_imp(void const*)
▒
+ 2.59% clang++ clang-3.8 [.]
llvm::LiveRange::extendInBlock(llvm::SlotIndex, llvm::SlotIndex)
▒
+ 2.27% clang++ clang-3.8 [.]
llvm::LiveRange::find(llvm::SlotIndex)
▒
+ 2.26% clang++ clang-3.8 [.] (anonymous
namespace)::InlineSpiller::traceSiblingValue(unsigned int, llvm::VNInfo*,
llvm::VNInfo*)
▒
+ 1.83% clang++ clang-3.8 [.]
llvm::BlockFrequency::operator*(llvm::BranchProbability const&) const
▒
+ 1.43% clang++ clang-3.8 [.]
extendSegmentsToUses(llvm::LiveRange&, llvm::SlotIndexes const&,
llvm::SmallVector<std::pair<llvm::SlotIndex, llvm::VNInfo*>, 16u>&,
llvm::LiveRange const&) ▒
+ 1.07% clang++ clang-3.8 [.]
llvm::BlockFrequency::operator*=(llvm::BranchProbability const&)
▒
+ 0.71% clang++ clang-3.8 [.]
llvm::MachineBasicBlock::isSuccessor(llvm::MachineBasicBlock const*) const
▒
+ 0.69% clang++ clang-3.8 [.]
llvm::ConnectedVNInfoEqClasses::Classify(llvm::LiveInterval const*)
▒
+ 0.56% clang++ [kernel.kallsyms] [k] 0xffffffff8104f45a
▒
+ 0.44% clang++ clang-3.8 [.]
llvm::DominatorTreeBase<llvm::MachineBasicBlock>::dominates(llvm::MachineBasicBlock
const*, llvm::MachineBasicBlock const*) const
▒
+ 0.43% clang++ libc-2.19.so [.] _int_malloc
▒
+ 0.36% clang++ clang-3.8 [.]
llvm::SlotIndexes::getMBBFromIndex(llvm::SlotIndex) const
▒
+ 0.35% clang++ libc-2.19.so [.] __memcpy_sse2_unaligned
▒
+ 0.32% clang++ clang-3.8 [.] llvm::Use::getImpliedUser() const
I have attached the .ii file. Below is the command to compile this file with
clang.
/usr/local/google2/cmtice/llvm-work/llvm-install.opt/bin/clang++ -c
-fno-exceptions -Wno-multichar -m64 -Wa,--noexecstack -fPIC
-no-canonical-prefixes -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -fstack-protector
-D__STDC_FORMAT_MACROS -D__STDC_CONSTANT_MACROS -DANDROID -fmessage-length=0 -W
-Wall -Wno-unused -Winit-self -Wpointer-arith -g -fno-strict-aliasing
-DNDEBUG -UDEBUG -D__compiler_offsetof=__builtin_offsetof
-Werror=int-conversion -Wno-reserved-id-macro -Wno-format-pedantic
-Wno-unused-command-line-argument -target x86_64-linux-gnu -DANDROID
-fmessage-length=0 -W -Wall -Wno-unused -Winit-self -Wpointer-arith
-Wsign-promo -DNDEBUG -UDEBUG -Wno-inconsistent-missing-override -target
x86_64-linux-gnu -DBUILDING_LIBART=1 -Wthread-safety -Wthread-safety-negative
-Wimplicit-fallthrough -Wfloat-equal -Wint-to-void-pointer-cast
-Wused-but-marked-unused -Wdeprecated -Wunreachable-code-break
-Wunreachable-code-return -Wmissing-noreturn -fno-omit-frame-pointer -fno-rtti
-std=gnu++11 -ggdb3 -Wall -Werror -Wextra -Wstrict-aliasing -fstrict-aliasing
-Wunreachable-code -Wredundant-decls -Wshadow -Wunused -fvisibility=protected
-DART_DEFAULT_GC_TYPE_IS_CMS -DIMT_SIZE=64 -DART_BASE_ADDRESS=0x60000000
-DART_DEFAULT_INSTRUCTION_SET_FEATURES=default
-DART_BASE_ADDRESS_MIN_DELTA=-0x1000000 -DART_BASE_ADDRESS_MAX_DELTA=0x1000000
-DART_DEFAULT_INSTRUCTION_SET_FEATURES="default" -O3 -Wframe-larger-than=2700
-fPIC -D_USING_LIBCXX -std=gnu++14 -nostdinc++ -Werror=int-to-pointer-cast
-Werror=pointer-to-int-cast -Werror=address-of-temporary
-Werror=null-dereference -Werror=return-type -o interpreter_goto_table_impl.o
./interpreter_goto_table_impl.ii
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