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198 lines
9.0 KiB
ReStructuredText
====================================================
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Using -opt-bisect-limit to debug optimization errors
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====================================================
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.. contents::
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:local:
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:depth: 1
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Introduction
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============
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The -opt-bisect-limit option provides a way to disable all optimization passes
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above a specified limit without modifying the way in which the Pass Managers
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are populated. The intention of this option is to assist in tracking down
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problems where incorrect transformations during optimization result in incorrect
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run-time behavior.
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This feature is implemented on an opt-in basis. Passes which can be safely
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skipped while still allowing correct code generation call a function to
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check the opt-bisect limit before performing optimizations. Passes which
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either must be run or do not modify the IR do not perform this check and are
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therefore never skipped. Generally, this means analysis passes, passes
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that are run at CodeGenOpt::None and passes which are required for register
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allocation.
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The -opt-bisect-limit option can be used with any tool, including front ends
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such as clang, that uses the core LLVM library for optimization and code
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generation. The exact syntax for invoking the option is discussed below.
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This feature is not intended to replace other debugging tools such as bugpoint.
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Rather it provides an alternate course of action when reproducing the problem
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requires a complex build infrastructure that would make using bugpoint
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impractical or when reproducing the failure requires a sequence of
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transformations that is difficult to replicate with tools like opt and llc.
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Getting Started
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===============
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The -opt-bisect-limit command line option can be passed directly to tools such
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as opt, llc and lli. The syntax is as follows:
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::
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<tool name> [other options] -opt-bisect-limit=<limit>
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If a value of -1 is used the tool will perform all optimizations but a message
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will be printed to stderr for each optimization that could be skipped
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indicating the index value that is associated with that optimization. To skip
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optimizations, pass the value of the last optimization to be performed as the
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opt-bisect-limit. All optimizations with a higher index value will be skipped.
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In order to use the -opt-bisect-limit option with a driver that provides a
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wrapper around the LLVM core library, an additional prefix option may be
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required, as defined by the driver. For example, to use this option with
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clang, the "-mllvm" prefix must be used. A typical clang invocation would look
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like this:
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::
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clang -O2 -mllvm -opt-bisect-limit=256 my_file.c
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The -opt-bisect-limit option may also be applied to link-time optimizations by
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using a prefix to indicate that this is a plug-in option for the linker. The
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following syntax will set a bisect limit for LTO transformations:
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::
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clang -flto -Wl,-plugin-opt,-opt-bisect-limit=256 my_file.o my_other_file.o
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LTO passes are run by a library instance invoked by the linker. Therefore any
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passes run in the primary driver compilation phase are not affected by options
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passed via '-Wl,-plugin-opt' and LTO passes are not affected by options
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passed to the driver-invoked LLVM invocation via '-mllvm'.
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Bisection Index Values
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======================
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The granularity of the optimizations associated with a single index value is
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variable. Depending on how the optimization pass has been instrumented the
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value may be associated with as much as all transformations that would have
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been performed by an optimization pass on an IR unit for which it is invoked
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(for instance, during a single call of runOnFunction for a FunctionPass) or as
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little as a single transformation. The index values may also be nested so that
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if an invocation of the pass is not skipped individual transformations within
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that invocation may still be skipped.
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The order of the values assigned is guaranteed to remain stable and consistent
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from one run to the next up to and including the value specified as the limit.
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Above the limit value skipping of optimizations can cause a change in the
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numbering, but because all optimizations above the limit are skipped this
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is not a problem.
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When an opt-bisect index value refers to an entire invocation of the run
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function for a pass, the pass will query whether or not it should be skipped
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each time it is invoked and each invocation will be assigned a unique value.
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For example, if a FunctionPass is used with a module containing three functions
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a different index value will be assigned to the pass for each of the functions
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as the pass is run. The pass may be run on two functions but skipped for the
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third.
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If the pass internally performs operations on a smaller IR unit the pass must be
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specifically instrumented to enable bisection at this finer level of granularity
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(see below for details).
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Example Usage
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=============
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.. code-block:: console
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$ opt -O2 -o test-opt.bc -opt-bisect-limit=16 test.ll
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BISECT: running pass (1) Simplify the CFG on function (g)
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BISECT: running pass (2) SROA on function (g)
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BISECT: running pass (3) Early CSE on function (g)
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BISECT: running pass (4) Infer set function attributes on module (test.ll)
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BISECT: running pass (5) Interprocedural Sparse Conditional Constant Propagation on module (test.ll)
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BISECT: running pass (6) Global Variable Optimizer on module (test.ll)
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BISECT: running pass (7) Promote Memory to Register on function (g)
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BISECT: running pass (8) Dead Argument Elimination on module (test.ll)
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BISECT: running pass (9) Combine redundant instructions on function (g)
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BISECT: running pass (10) Simplify the CFG on function (g)
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BISECT: running pass (11) Remove unused exception handling info on SCC (<<null function>>)
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BISECT: running pass (12) Function Integration/Inlining on SCC (<<null function>>)
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BISECT: running pass (13) Deduce function attributes on SCC (<<null function>>)
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BISECT: running pass (14) Remove unused exception handling info on SCC (f)
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BISECT: running pass (15) Function Integration/Inlining on SCC (f)
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BISECT: running pass (16) Deduce function attributes on SCC (f)
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BISECT: NOT running pass (17) Remove unused exception handling info on SCC (g)
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BISECT: NOT running pass (18) Function Integration/Inlining on SCC (g)
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BISECT: NOT running pass (19) Deduce function attributes on SCC (g)
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BISECT: NOT running pass (20) SROA on function (g)
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BISECT: NOT running pass (21) Early CSE on function (g)
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BISECT: NOT running pass (22) Speculatively execute instructions if target has divergent branches on function (g)
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... etc. ...
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Pass Skipping Implementation
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============================
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The -opt-bisect-limit implementation depends on individual passes opting in to
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the opt-bisect process. The OptBisect object that manages the process is
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entirely passive and has no knowledge of how any pass is implemented. When a
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pass is run if the pass may be skipped, it should call the OptBisect object to
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see if it should be skipped.
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The OptBisect object is intended to be accessed through LLVMContext and each
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Pass base class contains a helper function that abstracts the details in order
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to make this check uniform across all passes. These helper functions are:
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.. code-block:: c++
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bool ModulePass::skipModule(Module &M);
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bool CallGraphSCCPass::skipSCC(CallGraphSCC &SCC);
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bool FunctionPass::skipFunction(const Function &F);
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bool BasicBlockPass::skipBasicBlock(const BasicBlock &BB);
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bool LoopPass::skipLoop(const Loop *L);
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A MachineFunctionPass should use FunctionPass::skipFunction() as such:
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.. code-block:: c++
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bool MyMachineFunctionPass::runOnMachineFunction(Function &MF) {
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if (skipFunction(*MF.getFunction())
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return false;
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// Otherwise, run the pass normally.
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}
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In addition to checking with the OptBisect class to see if the pass should be
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skipped, the skipFunction(), skipLoop() and skipBasicBlock() helper functions
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also look for the presence of the "optnone" function attribute. The calling
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pass will be unable to determine whether it is being skipped because the
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"optnone" attribute is present or because the opt-bisect-limit has been
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reached. This is desirable because the behavior should be the same in either
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case.
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The majority of LLVM passes which can be skipped have already been instrumented
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in the manner described above. If you are adding a new pass or believe you
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have found a pass which is not being included in the opt-bisect process but
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should be, you can add it as described above.
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Adding Finer Granularity
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========================
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Once the pass in which an incorrect transformation is performed has been
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determined, it may be useful to perform further analysis in order to determine
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which specific transformation is causing the problem. Ideally all passes
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would be instrumented to allow skipping of individual transformations. This
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functionality is available through the OptBisect object but it is impractical
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to proactively instrument every existing pass. It is hoped that as developers
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find that they need a pass to be instrumented they will add the instrumentation
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and contribute it back to the LLVM source base.
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Helper functions will be added to simplify this level of instrumentation, but
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this work is not yet completed. For more information, contact Andy Kaylor.
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