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10 LLVM bugpoint tool: design and usage
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38 passes. It can be used to debug three types of failures: optimizer crashes,
39 miscompilations by optimizers, or bad native code generation (including problems
40 in the static and JIT compilers). It aims to reduce large test cases to small,
42 file, it will identify the optimization (or combination of optimizations) that
43 causes the crash, and reduce the file down to a small example which triggers the
44 crash.</p>
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61 hooks into the LLVM infrastructure at all. It works with any and all LLVM
62 passes and code generators, and does not need to "know" how they work. Because
63 of this, it may appear to do stupid things or miss obvious
65 time for computer time in the compiler-debugging process; consequently, it may
66 take a long period of (unattended) time to reduce a test case, but we feel it
68 debugging a miscompilation where each test of the program (which requires
69 executing it) takes a long time.</p>
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79 the command line and links them together into a single module, called the test
80 program. If any LLVM passes are specified on the command line, it runs these
81 passes on the test program. If any of the passes crash, or if they produce
88 a reference output for the test program, it tries executing it with the
89 selected code generator. If the selected code generator crashes,
91 code generator. Otherwise, if the resulting output differs from the reference
92 output, it assumes the difference resulted from a code generator failure, and
95 <p>Finally, if the output of the selected code generator matches the reference
97 have been applied to it. If its output differs from the reference output, it
98 assumes the difference resulted from a failure in one of the LLVM passes, and
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112 as it can to reduce the list of passes (for optimizer crashes) and the size of
114 optimizer passes triggers the bug. This is useful when debugging a problem
118 reduce its size. Usually it is able to reduce a test program to a single
119 function, when debugging intraprocedural optimizations. Once the number of
120 functions has been reduced, it attempts to delete various edges in the control
121 flow graph, to reduce the size of the function as much as possible. Finally,
124 passes crash, give you a bitcode file, and give you instructions on how to
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136 <p>The code generator debugger attempts to narrow down the amount of code that
137 is being miscompiled by the selected code generator. To do this, it takes the
138 test program and partitions it into two pieces: one piece which it compiles
139 with the C backend (into a shared object), and one piece which it runs with
140 either the JIT or the static LLC compiler. It uses several techniques to
141 reduce the amount of code pushed through the LLVM code generator, to reduce the
142 potential scope of the problem. After it is finished, it emits two bitcode
144 compiled with the C backend], respectively), and instructions for reproducing
145 the problem. The code generator debugger assumes that the C backend produces
146 good code.</p>
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157 <p>The miscompilation debugger works similarly to the code generator debugger.
158 It works by splitting the test program into two pieces, running the
159 optimizations specified on one piece, linking the two pieces back together, and
160 then executing the result. It attempts to narrow down the list of passes to
161 the one (or few) which are causing the miscompilation, then reduce the portion
162 of the test program which is being miscompiled. The miscompilation debugger
163 assumes that the selected code generator is working properly.</p>
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178 non-obvious ways. Here are some hints and tips:<p>
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182 works with programs that have deterministic output. Thus, if the program
185 as the result of a miscompilation. Programs should be temporarily modified
186 to disable outputs that are likely to vary from run to run.
188 <li>In the code generator and miscompilation debuggers, debugging will go
189 faster if you manually modify the program or its inputs to reduce the
190 runtime, but still exhibit the problem.
193 it helps track down regressions quickly. To avoid having to relink
199 of time. It is often useful to capture the output of the program to file.
200 For example, in the C shell, you can run:</p>
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212 that also crashes, you may be experiencing a linker bug.
216 the list of specified optimizations to be randomized and applied to the
217 program. This process will repeat until a bug is found or the user
222 can use e.g.</p>
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