1 # Assembly Tests 2 3 The Benchmark library provides a number of functions whose primary 4 purpose in to affect assembly generation, including `DoNotOptimize` 5 and `ClobberMemory`. In addition there are other functions, 6 such as `KeepRunning`, for which generating good assembly is paramount. 7 8 For these functions it's important to have tests that verify the 9 correctness and quality of the implementation. This requires testing 10 the code generated by the compiler. 11 12 This document describes how the Benchmark library tests compiler output, 13 as well as how to properly write new tests. 14 15 16 ## Anatomy of a Test 17 18 Writing a test has two steps: 19 20 * Write the code you want to generate assembly for. 21 * Add `// CHECK` lines to match against the verified assembly. 22 23 Example: 24 ```c++ 25 26 // CHECK-LABEL: test_add: 27 extern "C" int test_add() { 28 extern int ExternInt; 29 return ExternInt + 1; 30 31 // CHECK: movl ExternInt(%rip), %eax 32 // CHECK: addl %eax 33 // CHECK: ret 34 } 35 36 ``` 37 38 #### LLVM Filecheck 39 40 [LLVM's Filecheck](https://llvm.org/docs/CommandGuide/FileCheck.html) 41 is used to test the generated assembly against the `// CHECK` lines 42 specified in the tests source file. Please see the documentation 43 linked above for information on how to write `CHECK` directives. 44 45 #### Tips and Tricks: 46 47 * Tests should match the minimal amount of output required to establish 48 correctness. `CHECK` directives don't have to match on the exact next line 49 after the previous match, so tests should omit checks for unimportant 50 bits of assembly. ([`CHECK-NEXT`](https://llvm.org/docs/CommandGuide/FileCheck.html#the-check-next-directive) 51 can be used to ensure a match occurs exactly after the previous match). 52 53 * The tests are compiled with `-O3 -g0`. So we're only testing the 54 optimized output. 55 56 * The assembly output is further cleaned up using `tools/strip_asm.py`. 57 This removes comments, assembler directives, and unused labels before 58 the test is run. 59 60 * The generated and stripped assembly file for a test is output under 61 `<build-directory>/test/<test-name>.s` 62 63 * Filecheck supports using [`CHECK` prefixes](https://llvm.org/docs/CommandGuide/FileCheck.html#cmdoption-check-prefixes) 64 to specify lines that should only match in certain situations. 65 The Benchmark tests use `CHECK-CLANG` and `CHECK-GNU` for lines that 66 are only expected to match Clang or GCC's output respectively. Normal 67 `CHECK` lines match against all compilers. (Note: `CHECK-NOT` and 68 `CHECK-LABEL` are NOT prefixes. They are versions of non-prefixed 69 `CHECK` lines) 70 71 * Use `extern "C"` to disable name mangling for specific functions. This 72 makes them easier to name in the `CHECK` lines. 73 74 75 ## Problems Writing Portable Tests 76 77 Writing tests which check the code generated by a compiler are 78 inherently non-portable. Different compilers and even different compiler 79 versions may generate entirely different code. The Benchmark tests 80 must tolerate this. 81 82 LLVM Filecheck provides a number of mechanisms to help write 83 "more portable" tests; including [matching using regular expressions](https://llvm.org/docs/CommandGuide/FileCheck.html#filecheck-pattern-matching-syntax), 84 allowing the creation of [named variables](https://llvm.org/docs/CommandGuide/FileCheck.html#filecheck-variables) 85 for later matching, and [checking non-sequential matches](https://llvm.org/docs/CommandGuide/FileCheck.html#the-check-dag-directive). 86 87 #### Capturing Variables 88 89 For example, say GCC stores a variable in a register but Clang stores 90 it in memory. To write a test that tolerates both cases we "capture" 91 the destination of the store, and then use the captured expression 92 to write the remainder of the test. 93 94 ```c++ 95 // CHECK-LABEL: test_div_no_op_into_shr: 96 extern "C" void test_div_no_op_into_shr(int value) { 97 int divisor = 2; 98 benchmark::DoNotOptimize(divisor); // hide the value from the optimizer 99 return value / divisor; 100 101 // CHECK: movl $2, [[DEST:.*]] 102 // CHECK: idivl [[DEST]] 103 // CHECK: ret 104 } 105 ``` 106 107 #### Using Regular Expressions to Match Differing Output 108 109 Often tests require testing assembly lines which may subtly differ 110 between compilers or compiler versions. A common example of this 111 is matching stack frame addresses. In this case regular expressions 112 can be used to match the differing bits of output. For example: 113 114 ```c++ 115 int ExternInt; 116 struct Point { int x, y, z; }; 117 118 // CHECK-LABEL: test_store_point: 119 extern "C" void test_store_point() { 120 Point p{ExternInt, ExternInt, ExternInt}; 121 benchmark::DoNotOptimize(p); 122 123 // CHECK: movl ExternInt(%rip), %eax 124 // CHECK: movl %eax, -{{[0-9]+}}(%rsp) 125 // CHECK: movl %eax, -{{[0-9]+}}(%rsp) 126 // CHECK: movl %eax, -{{[0-9]+}}(%rsp) 127 // CHECK: ret 128 } 129 ``` 130 131 ## Current Requirements and Limitations 132 133 The tests require Filecheck to be installed along the `PATH` of the 134 build machine. Otherwise the tests will be disabled. 135 136 Additionally, as mentioned in the previous section, codegen tests are 137 inherently non-portable. Currently the tests are limited to: 138 139 * x86_64 targets. 140 * Compiled with GCC or Clang 141 142 Further work could be done, at least on a limited basis, to extend the 143 tests to other architectures and compilers (using `CHECK` prefixes). 144 145 Furthermore, the tests fail for builds which specify additional flags 146 that modify code generation, including `--coverage` or `-fsanitize=`. 147 148
