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 CodeGenOptLevel::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 78Passing ``-opt-bisect-print-ir-path=path/foo.ll`` will dump the IR to 79``path/foo.ll`` when -opt-bisect-limit starts skipping passes. 80 81Bisection Index Values 82====================== 83 84The granularity of the optimizations associated with a single index value is 85variable. Depending on how the optimization pass has been instrumented the 86value may be associated with as much as all transformations that would have 87been performed by an optimization pass on an IR unit for which it is invoked 88(for instance, during a single call of runOnFunction for a FunctionPass) or as 89little as a single transformation. The index values may also be nested so that 90if an invocation of the pass is not skipped individual transformations within 91that invocation may still be skipped. 92 93The order of the values assigned is guaranteed to remain stable and consistent 94from one run to the next up to and including the value specified as the limit. 95Above the limit value skipping of optimizations can cause a change in the 96numbering, but because all optimizations above the limit are skipped this 97is not a problem. 98 99When an opt-bisect index value refers to an entire invocation of the run 100function for a pass, the pass will query whether or not it should be skipped 101each time it is invoked and each invocation will be assigned a unique value. 102For example, if a FunctionPass is used with a module containing three functions 103a different index value will be assigned to the pass for each of the functions 104as the pass is run. The pass may be run on two functions but skipped for the 105third. 106 107If the pass internally performs operations on a smaller IR unit the pass must be 108specifically instrumented to enable bisection at this finer level of granularity 109(see below for details). 110 111 112Example Usage 113============= 114 115.. code-block:: console 116 117 $ opt -O2 -o test-opt.bc -opt-bisect-limit=16 test.ll 118 119 BISECT: running pass (1) Simplify the CFG on function (g) 120 BISECT: running pass (2) SROA on function (g) 121 BISECT: running pass (3) Early CSE on function (g) 122 BISECT: running pass (4) Infer set function attributes on module (test.ll) 123 BISECT: running pass (5) Interprocedural Sparse Conditional Constant Propagation on module (test.ll) 124 BISECT: running pass (6) Global Variable Optimizer on module (test.ll) 125 BISECT: running pass (7) Promote Memory to Register on function (g) 126 BISECT: running pass (8) Dead Argument Elimination on module (test.ll) 127 BISECT: running pass (9) Combine redundant instructions on function (g) 128 BISECT: running pass (10) Simplify the CFG on function (g) 129 BISECT: running pass (11) Remove unused exception handling info on SCC (<<null function>>) 130 BISECT: running pass (12) Function Integration/Inlining on SCC (<<null function>>) 131 BISECT: running pass (13) Deduce function attributes on SCC (<<null function>>) 132 BISECT: running pass (14) Remove unused exception handling info on SCC (f) 133 BISECT: running pass (15) Function Integration/Inlining on SCC (f) 134 BISECT: running pass (16) Deduce function attributes on SCC (f) 135 BISECT: NOT running pass (17) Remove unused exception handling info on SCC (g) 136 BISECT: NOT running pass (18) Function Integration/Inlining on SCC (g) 137 BISECT: NOT running pass (19) Deduce function attributes on SCC (g) 138 BISECT: NOT running pass (20) SROA on function (g) 139 BISECT: NOT running pass (21) Early CSE on function (g) 140 BISECT: NOT running pass (22) Speculatively execute instructions if target has divergent branches on function (g) 141 ... etc. ... 142 143 144Pass Skipping Implementation 145============================ 146 147The -opt-bisect-limit implementation depends on individual passes opting in to 148the opt-bisect process. The OptBisect object that manages the process is 149entirely passive and has no knowledge of how any pass is implemented. When a 150pass is run if the pass may be skipped, it should call the OptBisect object to 151see if it should be skipped. 152 153The OptBisect object is intended to be accessed through LLVMContext and each 154Pass base class contains a helper function that abstracts the details in order 155to make this check uniform across all passes. These helper functions are: 156 157.. code-block:: c++ 158 159 bool ModulePass::skipModule(Module &M); 160 bool CallGraphSCCPass::skipSCC(CallGraphSCC &SCC); 161 bool FunctionPass::skipFunction(const Function &F); 162 bool LoopPass::skipLoop(const Loop *L); 163 164A MachineFunctionPass should use FunctionPass::skipFunction() as such: 165 166.. code-block:: c++ 167 168 bool MyMachineFunctionPass::runOnMachineFunction(Function &MF) { 169 if (skipFunction(*MF.getFunction()) 170 return false; 171 // Otherwise, run the pass normally. 172 } 173 174In addition to checking with the OptBisect class to see if the pass should be 175skipped, the skipFunction(), skipLoop() and skipBasicBlock() helper functions 176also look for the presence of the "optnone" function attribute. The calling 177pass will be unable to determine whether it is being skipped because the 178"optnone" attribute is present or because the opt-bisect-limit has been 179reached. This is desirable because the behavior should be the same in either 180case. 181 182The majority of LLVM passes which can be skipped have already been instrumented 183in the manner described above. If you are adding a new pass or believe you 184have found a pass which is not being included in the opt-bisect process but 185should be, you can add it as described above. 186 187 188Adding Finer Granularity 189======================== 190 191Once the pass in which an incorrect transformation is performed has been 192determined, it may be useful to perform further analysis in order to determine 193which specific transformation is causing the problem. Debug counters 194can be used for this purpose. 195