1==================================================== 2Using -opt-bisect-limit to debug optimization errors 3==================================================== 4.. contents:: 5 :local: 6 :depth: 1 7 8Introduction 9============ 10 11The -opt-bisect-limit option provides a way to disable all optimization passes 12above a specified limit without modifying the way in which the Pass Managers 13are populated. The intention of this option is to assist in tracking down 14problems where incorrect transformations during optimization result in incorrect 15run-time behavior. 16 17This feature is implemented on an opt-in basis. Passes which can be safely 18skipped while still allowing correct code generation call a function to 19check the opt-bisect limit before performing optimizations. Passes which 20either must be run or do not modify the IR do not perform this check and are 21therefore never skipped. Generally, this means analysis passes, passes 22that are run at CodeGenOpt::None and passes which are required for register 23allocation. 24 25The -opt-bisect-limit option can be used with any tool, including front ends 26such as clang, that uses the core LLVM library for optimization and code 27generation. The exact syntax for invoking the option is discussed below. 28 29This feature is not intended to replace other debugging tools such as bugpoint. 30Rather it provides an alternate course of action when reproducing the problem 31requires a complex build infrastructure that would make using bugpoint 32impractical or when reproducing the failure requires a sequence of 33transformations that is difficult to replicate with tools like opt and llc. 34 35 36Getting Started 37=============== 38 39The -opt-bisect-limit command line option can be passed directly to tools such 40as opt, llc and lli. The syntax is as follows: 41 42:: 43 44 <tool name> [other options] -opt-bisect-limit=<limit> 45 46If a value of -1 is used the tool will perform all optimizations but a message 47will be printed to stderr for each optimization that could be skipped 48indicating the index value that is associated with that optimization. To skip 49optimizations, pass the value of the last optimization to be performed as the 50opt-bisect-limit. All optimizations with a higher index value will be skipped. 51 52In order to use the -opt-bisect-limit option with a driver that provides a 53wrapper around the LLVM core library, an additional prefix option may be 54required, as defined by the driver. For example, to use this option with 55clang, the "-mllvm" prefix must be used. A typical clang invocation would look 56like this: 57 58:: 59 60 clang -O2 -mllvm -opt-bisect-limit=256 my_file.c 61 62The -opt-bisect-limit option may also be applied to link-time optimizations by 63using a prefix to indicate that this is a plug-in option for the linker. The 64following syntax will set a bisect limit for LTO transformations: 65 66:: 67 68 # When using lld, or ld64 (macOS) 69 clang -flto -Wl,-mllvm,-opt-bisect-limit=256 my_file.o my_other_file.o 70 # When using Gold 71 clang -flto -Wl,-plugin-opt,-opt-bisect-limit=256 my_file.o my_other_file.o 72 73LTO passes are run by a library instance invoked by the linker. Therefore any 74passes run in the primary driver compilation phase are not affected by options 75passed via '-Wl,-plugin-opt' and LTO passes are not affected by options 76passed to the driver-invoked LLVM invocation via '-mllvm'. 77 78 79Bisection Index Values 80====================== 81 82The granularity of the optimizations associated with a single index value is 83variable. Depending on how the optimization pass has been instrumented the 84value may be associated with as much as all transformations that would have 85been performed by an optimization pass on an IR unit for which it is invoked 86(for instance, during a single call of runOnFunction for a FunctionPass) or as 87little as a single transformation. The index values may also be nested so that 88if an invocation of the pass is not skipped individual transformations within 89that invocation may still be skipped. 90 91The order of the values assigned is guaranteed to remain stable and consistent 92from one run to the next up to and including the value specified as the limit. 93Above the limit value skipping of optimizations can cause a change in the 94numbering, but because all optimizations above the limit are skipped this 95is not a problem. 96 97When an opt-bisect index value refers to an entire invocation of the run 98function for a pass, the pass will query whether or not it should be skipped 99each time it is invoked and each invocation will be assigned a unique value. 100For example, if a FunctionPass is used with a module containing three functions 101a different index value will be assigned to the pass for each of the functions 102as the pass is run. The pass may be run on two functions but skipped for the 103third. 104 105If the pass internally performs operations on a smaller IR unit the pass must be 106specifically instrumented to enable bisection at this finer level of granularity 107(see below for details). 108 109 110Example Usage 111============= 112 113.. code-block:: console 114 115 $ opt -O2 -o test-opt.bc -opt-bisect-limit=16 test.ll 116 117 BISECT: running pass (1) Simplify the CFG on function (g) 118 BISECT: running pass (2) SROA on function (g) 119 BISECT: running pass (3) Early CSE on function (g) 120 BISECT: running pass (4) Infer set function attributes on module (test.ll) 121 BISECT: running pass (5) Interprocedural Sparse Conditional Constant Propagation on module (test.ll) 122 BISECT: running pass (6) Global Variable Optimizer on module (test.ll) 123 BISECT: running pass (7) Promote Memory to Register on function (g) 124 BISECT: running pass (8) Dead Argument Elimination on module (test.ll) 125 BISECT: running pass (9) Combine redundant instructions on function (g) 126 BISECT: running pass (10) Simplify the CFG on function (g) 127 BISECT: running pass (11) Remove unused exception handling info on SCC (<<null function>>) 128 BISECT: running pass (12) Function Integration/Inlining on SCC (<<null function>>) 129 BISECT: running pass (13) Deduce function attributes on SCC (<<null function>>) 130 BISECT: running pass (14) Remove unused exception handling info on SCC (f) 131 BISECT: running pass (15) Function Integration/Inlining on SCC (f) 132 BISECT: running pass (16) Deduce function attributes on SCC (f) 133 BISECT: NOT running pass (17) Remove unused exception handling info on SCC (g) 134 BISECT: NOT running pass (18) Function Integration/Inlining on SCC (g) 135 BISECT: NOT running pass (19) Deduce function attributes on SCC (g) 136 BISECT: NOT running pass (20) SROA on function (g) 137 BISECT: NOT running pass (21) Early CSE on function (g) 138 BISECT: NOT running pass (22) Speculatively execute instructions if target has divergent branches on function (g) 139 ... etc. ... 140 141 142Pass Skipping Implementation 143============================ 144 145The -opt-bisect-limit implementation depends on individual passes opting in to 146the opt-bisect process. The OptBisect object that manages the process is 147entirely passive and has no knowledge of how any pass is implemented. When a 148pass is run if the pass may be skipped, it should call the OptBisect object to 149see if it should be skipped. 150 151The OptBisect object is intended to be accessed through LLVMContext and each 152Pass base class contains a helper function that abstracts the details in order 153to make this check uniform across all passes. These helper functions are: 154 155.. code-block:: c++ 156 157 bool ModulePass::skipModule(Module &M); 158 bool CallGraphSCCPass::skipSCC(CallGraphSCC &SCC); 159 bool FunctionPass::skipFunction(const Function &F); 160 bool BasicBlockPass::skipBasicBlock(const BasicBlock &BB); 161 bool LoopPass::skipLoop(const Loop *L); 162 163A MachineFunctionPass should use FunctionPass::skipFunction() as such: 164 165.. code-block:: c++ 166 167 bool MyMachineFunctionPass::runOnMachineFunction(Function &MF) { 168 if (skipFunction(*MF.getFunction()) 169 return false; 170 // Otherwise, run the pass normally. 171 } 172 173In addition to checking with the OptBisect class to see if the pass should be 174skipped, the skipFunction(), skipLoop() and skipBasicBlock() helper functions 175also look for the presence of the "optnone" function attribute. The calling 176pass will be unable to determine whether it is being skipped because the 177"optnone" attribute is present or because the opt-bisect-limit has been 178reached. This is desirable because the behavior should be the same in either 179case. 180 181The majority of LLVM passes which can be skipped have already been instrumented 182in the manner described above. If you are adding a new pass or believe you 183have found a pass which is not being included in the opt-bisect process but 184should be, you can add it as described above. 185 186 187Adding Finer Granularity 188======================== 189 190Once the pass in which an incorrect transformation is performed has been 191determined, it may be useful to perform further analysis in order to determine 192which specific transformation is causing the problem. Debug counters 193can be used for this purpose. 194