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This patch contains the basic functionality for reporting potentially incorrect usage of __builtin_expect() by comparing the developer's annotation against a collected PGO profile. A more detailed proposal and discussion appears on the CFE-dev mailing list (http://lists.llvm.org/pipermail/cfe-dev/2019-July/062971.html) and a prototype of the initial frontend changes appear here in D65300 We revised the work in D65300 by moving the misexpect check into the LLVM backend, and adding support for IR and sampling based profiles, in addition to frontend instrumentation. We add new misexpect metadata tags to those instructions directly influenced by the llvm.expect intrinsic (branch, switch, and select) when lowering the intrinsics. The misexpect metadata contains information about the expected target of the intrinsic so that we can check against the correct PGO counter when emitting diagnostics, and the compiler's values for the LikelyBranchWeight and UnlikelyBranchWeight. We use these branch weight values to determine when to emit the diagnostic to the user. A future patch should address the comment at the top of LowerExpectIntrisic.cpp to hoist the LikelyBranchWeight and UnlikelyBranchWeight values into a shared space that can be accessed outside of the LowerExpectIntrinsic pass. Once that is done, the misexpect metadata can be updated to be smaller. In the long term, it is possible to reconstruct portions of the misexpect metadata from the existing profile data. However, we have avoided this to keep the code simple, and because some kind of metadata tag will be required to identify which branch/switch/select instructions are influenced by the use of llvm.expect Patch By: paulkirth Differential Revision: https://reviews.llvm.org/D66324 llvm-svn: 371635
44 lines
1.8 KiB
C++
44 lines
1.8 KiB
C++
//===--- MisExpect.h - Check the use of llvm.expect with PGO data ---------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This contains code to emit warnings for potentially incorrect usage of the
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// llvm.expect intrinsic. This utility extracts the threshold values from
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// metadata associated with the instrumented Branch or Switch instruction. The
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// threshold values are then used to determine if a warning should be emmited.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/LLVMContext.h"
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namespace llvm {
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namespace misexpect {
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/// verifyMisExpect - compares PGO counters to the thresholds used for
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/// llvm.expect and warns if the PGO counters are outside of the expected
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/// range.
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/// \param I The Instruction being checked
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/// \param Weights A vector of profile weights for each target block
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/// \param Ctx The current LLVM context
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void verifyMisExpect(llvm::Instruction *I,
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const llvm::SmallVector<uint32_t, 4> &Weights,
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llvm::LLVMContext &Ctx);
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/// checkClangInstrumentation - verify if llvm.expect matches PGO profile
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/// This function checks the frontend instrumentation in the backend when
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/// lowering llvm.expect intrinsics. It checks for existing metadata, and
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/// then validates the use of llvm.expect against the assigned branch weights.
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//
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/// \param I the Instruction being checked
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void checkFrontendInstrumentation(Instruction &I);
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} // namespace misexpect
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} // namespace llvm
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