Differential D101970
[X86FixupLEAs] Transform the sequence LEA/SUB to SUB/SUB Authored by Carrot on May 5 2021, 7:43 PM.
Details This patch transforms the sequence lea (reg1, reg2), reg3
sub reg3, reg4
to two sub instructions
sub reg1, reg4
sub reg2, reg4Similar optimization can also be applied to LEA/ADD sequence.
Diff Detail
Event Timeline
Comment Actions I think it is good when the patch's description not just what the patch does, but also why it does that. Comment Actions Same as other LEA -> ALU optimizations in optTwoAddrLEA. On old architectures such as HSW or SKL, there are less issue ports containing LEA function unit than ALU. So LEA instructions may be delayed due to issue port competition. On newer architectures such as ICL or TGL, all four ports can issue LEA instructions, there is no such problem, but it doesn't hurt, because all optimizations in optTwoAddrLEA don't generate extra instructions. Comment Actions I studied the page for a while, but couldn't find the function to test the compile time of a patch, I can only find the comparison between two commits. Comment Actions https://llvm-compile-time-tracker.com/about.php Assuming you have a public llvm fork on your github, @nikic can set you up - you'd need to create a branch with 'perf/' and it would get picked up automatically. @xbolva00 what in particular are you concerned about? Avoiding LEA can be useful but I doubt there will be much effect here. Comment Actions Whether we want to run this transformation on newer archs - as you said, it brings us nothing, so atleast we should check it is free in terms of compile time in the backend. Comment Actions Compile-time: https://llvm-compile-time-tracker.com/compare.php?from=15565403722ec37d8b1a3ee8625ee2e8efcd96ee&to=ebe267e3e1133d04a80a40357ebe93a2fe6b018b&stat=instructions Looks neutral, the NewPM-O3 SPASS regressions is most likely just noise.
Comment Actions Just a heads up, my auto-bisecting multi-stage cron job has identified this change as the source of a second stage regression in a bunch of clang unit tests. I'm about to verify by hand, but it'll take a while. Comment Actions Lots of crashes like this: FAIL: Clang-Unit :: Format/./FormatTests/FormatTest.FormatsCompactNamespaces (23265 of 76779) ******************** TEST 'Clang-Unit :: Format/./FormatTests/FormatTest.FormatsCompactNamespaces' FAILED ******************** Note: Google Test filter = FormatTest.FormatsCompactNamespaces [==========] Running 1 test from 1 test suite. [----------] Global test environment set-up. [----------] 1 test from FormatTest [ RUN ] FormatTest.FormatsCompactNamespaces FormatTests: /home/dave/ro_s/lp/clang/lib/Basic/SourceManager.cpp:865: clang::FileID clang::SourceManager::getFileIDLoaded(unsigned int) const: Assertion `0 && "Invalid SLocOffset or bad function choice"' failed. #0 0x0000000000604313 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x604313) #1 0x0000000000602962 llvm::sys::RunSignalHandlers() (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x602962) #2 0x0000000000604d8a SignalHandler(int) Signals.cpp:0:0 #3 0x00007ffff7fa4a20 (/lib64/libpthread.so.0+0x13a20) #4 0x00007ffff7b392a2 raise (/lib64/libc.so.6+0x3d2a2) #5 0x00007ffff7b228a4 abort (/lib64/libc.so.6+0x268a4) #6 0x00007ffff7b22789 (/lib64/libc.so.6+0x26789) #7 0x00007ffff7b31a16 (/lib64/libc.so.6+0x35a16) #8 0x000000000064922c clang::SourceManager::getFileIDLoaded(unsigned int) const (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x64922c) #9 0x00000000003e734c clang::SourceManager::getDecomposedLoc(clang::SourceLocation) const CleanupTest.cpp:0:0 #10 0x000000000064b231 clang::SourceManager::isBeforeInTranslationUnit(clang::SourceLocation, clang::SourceLocation) const (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x64b231) #11 0x000000000066c05e clang::format::AffectedRangeManager::computeAffectedLines(llvm::SmallVectorImpl<clang::format::AnnotatedLine*>&) (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x66c05e) #12 0x00000000006ae393 clang::format::UsingDeclarationsSorter::analyze(clang::format::TokenAnnotator&, llvm::SmallVectorImpl<clang::format::AnnotatedLine*>&, clang::format::FormatTokenLexer&) (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x6ae393) #13 0x0000000000690d1a clang::format::TokenAnalyzer::process() (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x690d1a) #14 0x0000000000665643 std::__1::__function::__func<clang::format::internal::reformat(clang::format::FormatStyle const&, llvm::StringRef, llvm::ArrayRef<clang::tooling::Range>, unsigned int, unsigned int, unsigned int, llvm::StringRef, clang::format::FormattingAttemptStatus*)::$_4, std::__1::allocator<clang::format::internal::reformat(clang::format::FormatStyle const&, llvm::StringRef, llvm::ArrayRef<clang::tooling::Range>, unsigned int, unsigned int, unsigned int, llvm::StringRef, clang::format::FormattingAttemptStatus*)::$_4>, std::__1::pair<clang::tooling::Replacements, unsigned int> (clang::format::Environment const&)>::operator()(clang::format::Environment const&) Format.cpp:0:0 #15 0x0000000000655d6e clang::format::internal::reformat(clang::format::FormatStyle const&, llvm::StringRef, llvm::ArrayRef<clang::tooling::Range>, unsigned int, unsigned int, unsigned int, llvm::StringRef, clang::format::FormattingAttemptStatus*) (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x655d6e) #16 0x000000000065640f clang::format::reformat(clang::format::FormatStyle const&, llvm::StringRef, llvm::ArrayRef<clang::tooling::Range>, llvm::StringRef, clang::format::FormattingAttemptStatus*) (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x65640f) #17 0x00000000003ef27f clang::format::(anonymous namespace)::FormatTest::format(llvm::StringRef, clang::format::FormatStyle const&, clang::format::(anonymous namespace)::FormatTest::StatusCheck) FormatTest.cpp:0:0 #18 0x0000000000407eed clang::format::(anonymous namespace)::FormatTest_FormatsCompactNamespaces_Test::TestBody() FormatTest.cpp:0:0 #19 0x000000000060cc53 testing::Test::Run() (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x60cc53) #20 0x000000000060dc16 testing::TestInfo::Run() (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x60dc16) #21 0x000000000060e470 testing::TestSuite::Run() (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x60e470) #22 0x000000000061bb23 testing::internal::UnitTestImpl::RunAllTests() (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x61bb23) #23 0x000000000061b59d testing::UnitTest::Run() (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x61b59d) #24 0x000000000060555b main (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x60555b) #25 0x00007ffff7b23b75 __libc_start_main (/lib64/libc.so.6+0x27b75) #26 0x00000000003ddd8e _start (/tmp/_update_lc/t/tools/clang/unittests/Format/./FormatTests+0x3ddd8e) Comment Actions Verified. Second stage miscompile. If it matters (and it sometimes does), my first stage is built without asserts. Comment Actions Thanks! Also, I feel fairly confident about this. My auto-bisecting cron job is really paranoid. For example, it always does a multi-stage test to verify that the commit *before* the commit blamed by git-bisect actually works. That's what I was waiting for. Also, I then verified by trying to move one commit ahead in the history to this commit to get an A/B test and I then confirmed that this is the regression. Thanks again for reverting this. Comment Actions Thanks for the report! The problem is in - lea (%rax,%rcx,1),%ebp
mov %esi,%edi
- sub %ebp,%edi
- jbe <_ZNK5clang7tooling12Replacements5mergeERKS1_+0x279>
+ sub %eax,%edi
+ sub %ecx,%edi
+ jbe <_ZNK5clang7tooling12Replacements5mergeERKS1_+0x278>Previously I thought X - (Y + Z) generates same flags as X - Y - Z, unfortunately it is not true when X - Y overflows. This patch should fix the problem. @RKSimon, could you please take another look. @davezarzycki, could you help to test this patch with your configuration? thanks! Comment Actions I can check it, but I'm a bit concerned about the comment and what it implies. The problem is NOT overflow, but that comparison operations need to be redistributed during this optimization (or dependent comparisons need to disable the optimization). So in pseudocode, the original code that miscompiled looks like this: if (x - (y + z) <= 0) goto something; // where <= is a unsigned comparison but it really doesn't matter. So redistributing the arithmetic requires redistributing the comparison. auto t1 = x - y Does the new patch redistribute comparisons? Comment Actions Actually, it is more complicated, and you're right that it also can involve overflow. I'm not sure if this is a trivial optimization when dependent comparisons are involved. Comment Actions The new changes in function searchALUInst exactly does this check. // X - (Y + Z) may generate different flags than (X - Y) - Z when there
// is overflow. So we can't change the alu instruction if the flags
// register is live.
MachineBasicBlock::iterator NextI = std::next(CurInst);
if (MBB.computeRegisterLiveness(TRI, X86::EFLAGS, NextI) !=
MachineBasicBlock::LQR_Dead)
return MachineBasicBlock::iterator();If the flags of SUB/ADD is live, it will be used by following comparison or conditional branch or other instructions, we will give up the transformation. In other words, only when the flags of the ALU instruction will not be used by following instructions (comparison or conditional branch), then we can do the transformation. Comment Actions I've verified that this patch no longer causes my multi-stage cron job to regress. Thanks!
Comment Actions Check the dead EFLAGS def directly instead of calling computeRegisterLiveness.
Comment Actions We are still observing a miscompile caused by the newer version of this patch. I'll try to provide you a repro. Comment Actions Filed https://bugs.llvm.org/show_bug.cgi?id=50615. The original repro is in Java, but I hope the logs there are enough to figure out what's going on. The log shows that the described transform did apply. I suggest to revert and then investigate. Comment Actions Attached repro runnable with upstream llc to bug. Seems that it reproduces the same effect. Comment Actions Thanks for the report! Comment Actions I'm a bit surprised that it wasn't reverted. The default policy is revert whatever is broken and then fix it. We have numerous internal failures & broken CI cycles because of this, and the fix is not merged yet (and not clear when it will be merged). @Carrot could you please revert this and return back along with fix? Comment Actions I also suspect this patch is causing the following failure (with -mllvm -verify-machineinstrs ) on GreenDragon (http://green.lab.llvm.org/green/job/test-suite-verify-machineinstrs-x86_64-O3/9579/) /Users/buildslave/jenkins/workspace/test-suite-verify-machineinstrs-x86_64-O3/test-suite-build/tools/timeit --summary MultiSource/Applications/JM/ldecod/CMakeFiles/ldecod.dir/block.c.o.time /Users/buildslave/jenkins/workspace/test-suite-verify-machineinstrs-x86_64-O3/compiler/bin/clang -DNDEBUG -B /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin -Wno-unused-command-line-argument -mllvm -verify-machineinstrs -O3 -arch x86_64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -w -Werror=date-time -fcommon -D__USE_LARGEFILE64 -D_FILE_OFFSET_BITS=64 -MD -MT MultiSource/Applications/JM/ldecod/CMakeFiles/ldecod.dir/block.c.o -MF MultiSource/Applications/JM/ldecod/CMakeFiles/ldecod.dir/block.c.o.d -o MultiSource/Applications/JM/ldecod/CMakeFiles/ldecod.dir/block.c.o -c /Users/buildslave/jenkins/workspace/test-suite-verify-machineinstrs-x86_64-O3/test-suite/MultiSource/Applications/JM/ldecod/block.c Fatal Error: error in backend: Found 2 machine code errors. *** Bad machine code: Using an undefined physical register *** - function: itrans_sp - basic block: %bb.0 entry (0x7fe93e957ac8) - instruction: $ecx = SUB32rr $ecx(tied-def 0), $esi, implicit-def dead $eflags - operand 2: $esi fatal error: error in backend: Found 2 machine code errors. PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace, preprocessed source, and associated run script. Stack dump: Error: clang frontend command failed with exit code 70 (use -v to see invocation) 25 clang 0x000000011115a63d clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&) const + 221 26 clang 0x000000011115ab9d clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::__1::pair<int, clang::driver::Command const*> >&) const + 125 27 clang 0x00000001111715ec clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::__1::pair<int, clang::driver::Command const*> >&) + 204 28 clang 0x000000010e89a145 main + 10309 29 libdyld.dylib 0x00007fff67a6fcc9 start + 1 clang-13: error: clang frontend command failed with exit code 70 (use -v to see invocation) clang version 13.0.0 (https://github.com/llvm/llvm-project.git f0a68bbc967ab851e9b678feaf9015a2bfadb12e) Target: x86_64-apple-darwin19.5.0 It would be great if you could take a look and revert the patch if the investigation will take longer. Comment Actions Unfortunately this still creates invalid Machine IR for the llvm-test-suite on X86, as I mentioned above. E.g. see http://green.lab.llvm.org/green/job/test-suite-verify-machineinstrs-x86_64-O3/9585/ I reverted the fix and the patch in 5cd66420ccb1, 1b748faf2bae to get the public bots back to green. To reproduce, llc -verify-machineinstrs on the IR below on X86: target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
%struct.widget = type { i32, i32, i32, i32, i32*, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, [16 x [16 x i16]], [6 x [32 x i32]], [16 x [16 x i32]], [4 x [12 x [4 x [4 x i32]]]], [16 x i32], i8**, i32*, i32***, i32**, i32, i32, i32, i32, %struct.baz*, %struct.wobble.1*, i32, i32, i32, i32, i32, i32, %struct.quux.2*, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, [3 x i32], i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32***, i32***, i32****, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, [3 x [2 x i32]], [3 x [2 x i32]], i32, i32, i64, i64, %struct.zot.3, %struct.zot.3, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32 }
%struct.baz = type { i32, i32, i32, i32, i32, i32, i32, i32, i32, %struct.snork*, %struct.wombat.0*, %struct.wobble*, i32, i32*, i32*, i32*, i32, i32*, i32*, i32*, i32 (%struct.widget*, %struct.eggs*)*, i32, i32, i32, i32 }
%struct.snork = type { %struct.spam*, %struct.zot, i32 (%struct.wombat*, %struct.widget*, %struct.snork*)* }
%struct.spam = type { i32, i32, i32, i32, i8*, i32 }
%struct.zot = type { i32, i32, i32, i32, i32, i8*, i32* }
%struct.wombat = type { i32, i32, i32, i32, i32, i32, i32, i32, void (i32, i32, i32*, i32*)*, void (%struct.wombat*, %struct.widget*, %struct.zot*)* }
%struct.wombat.0 = type { [4 x [11 x %struct.quux]], [2 x [9 x %struct.quux]], [2 x [10 x %struct.quux]], [2 x [6 x %struct.quux]], [4 x %struct.quux], [4 x %struct.quux], [3 x %struct.quux] }
%struct.quux = type { i16, i8 }
%struct.wobble = type { [2 x %struct.quux], [4 x %struct.quux], [3 x [4 x %struct.quux]], [10 x [4 x %struct.quux]], [10 x [15 x %struct.quux]], [10 x [15 x %struct.quux]], [10 x [5 x %struct.quux]], [10 x [5 x %struct.quux]], [10 x [15 x %struct.quux]], [10 x [15 x %struct.quux]] }
%struct.eggs = type { [1000 x i8], [1000 x i8], [1000 x i8], i32, i32, i32, i32, i32, i32, i32, i32 }
%struct.wobble.1 = type { i32, [2 x i32], i32, i32, %struct.wobble.1*, %struct.wobble.1*, i32, [2 x [4 x [4 x [2 x i32]]]], i32, i64, i64, i32, i32, [4 x i8], [4 x i8], i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32 }
%struct.quux.2 = type { i32, i32, i32, i32, i32, %struct.quux.2* }
%struct.zot.3 = type { i64, i16, i16, i16 }
define void @blam(%struct.widget* %arg, i32 %arg1) local_unnamed_addr {
bb:
%tmp = load i32, i32* undef, align 4
%tmp2 = sdiv i32 %tmp, 6
%tmp3 = sdiv i32 undef, 6
%tmp4 = load i32, i32* undef, align 4
%tmp5 = icmp eq i32 %tmp4, 4
%tmp6 = select i1 %tmp5, i32 %tmp3, i32 %tmp2
%tmp7 = getelementptr inbounds [4 x [4 x i32]], [4 x [4 x i32]]* undef, i64 0, i64 0, i64 0
%tmp8 = zext i16 undef to i32
%tmp9 = zext i16 undef to i32
%tmp10 = load i16, i16* undef, align 2
%tmp11 = zext i16 %tmp10 to i32
%tmp12 = zext i16 undef to i32
%tmp13 = zext i16 undef to i32
%tmp14 = zext i16 undef to i32
%tmp15 = load i16, i16* undef, align 2
%tmp16 = zext i16 %tmp15 to i32
%tmp17 = zext i16 undef to i32
%tmp18 = sub nsw i32 %tmp8, %tmp9
%tmp19 = shl nsw i32 undef, 1
%tmp20 = add nsw i32 %tmp19, %tmp18
%tmp21 = sub nsw i32 %tmp11, %tmp12
%tmp22 = shl nsw i32 undef, 1
%tmp23 = add nsw i32 %tmp22, %tmp21
%tmp24 = sub nsw i32 %tmp13, %tmp14
%tmp25 = shl nsw i32 undef, 1
%tmp26 = add nsw i32 %tmp25, %tmp24
%tmp27 = sub nsw i32 %tmp16, %tmp17
%tmp28 = shl nsw i32 undef, 1
%tmp29 = add nsw i32 %tmp28, %tmp27
%tmp30 = sub nsw i32 %tmp20, %tmp29
%tmp31 = sub nsw i32 %tmp23, %tmp26
%tmp32 = shl nsw i32 %tmp30, 1
%tmp33 = add nsw i32 %tmp32, %tmp31
store i32 %tmp33, i32* undef, align 4
%tmp34 = mul nsw i32 %tmp31, -2
%tmp35 = add nsw i32 %tmp34, %tmp30
store i32 %tmp35, i32* undef, align 4
%tmp36 = select i1 %tmp5, i32 undef, i32 undef
br label %bb37
bb37: ; preds = %bb
%tmp38 = load i32, i32* undef, align 4
%tmp39 = ashr i32 %tmp38, %tmp6
%tmp40 = load i32, i32* undef, align 4
%tmp41 = sdiv i32 %tmp39, %tmp40
store i32 %tmp41, i32* undef, align 4
ret void
}Comment Actions Thanks for the test case! -- Before transformation renamable $esi = LEA64_32r renamable $rcx, 1, renamable $rcx, 0, $noreg renamable $edi = LEA64_32r renamable $rcx, 2, renamable $rcx, 0, $noreg $eax = MOV32rr $ecx, implicit killed $rcx // $rcx is killed at here renamable $eax = SUB32rr killed renamable $eax(tied-def 0), killed renamable $esi, implicit-def dead $eflags -- After transformation renamable $edi = LEA64_32r renamable $rcx, 2, renamable $rcx, 0, $noreg $eax = MOV32rr $ecx, implicit killed $rcx // $rcx should not be killed at here $eax = SUB32rr $eax(tied-def 0), $ecx, implicit-def dead $eflags $eax = SUB32rr $eax(tied-def 0), $ecx, implicit-def dead $eflags // $rcx should be killed at here The transformation may extend the life range of original BaseReg and IndexReg like $rcx in this test case, so the original kill flag should be cleared, and new kill flag should be added in new instruction. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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