7bdad08429
This patch contains the basic functionality for reporting potentially incorrect usage of __builtin_expect() by comparing the developer's annotation against a collected PGO profile. A more detailed proposal and discussion appears on the CFE-dev mailing list (http://lists.llvm.org/pipermail/cfe-dev/2019-July/062971.html) and a prototype of the initial frontend changes appear here in D65300 We revised the work in D65300 by moving the misexpect check into the LLVM backend, and adding support for IR and sampling based profiles, in addition to frontend instrumentation. We add new misexpect metadata tags to those instructions directly influenced by the llvm.expect intrinsic (branch, switch, and select) when lowering the intrinsics. The misexpect metadata contains information about the expected target of the intrinsic so that we can check against the correct PGO counter when emitting diagnostics, and the compiler's values for the LikelyBranchWeight and UnlikelyBranchWeight. We use these branch weight values to determine when to emit the diagnostic to the user. A future patch should address the comment at the top of LowerExpectIntrisic.cpp to hoist the LikelyBranchWeight and UnlikelyBranchWeight values into a shared space that can be accessed outside of the LowerExpectIntrinsic pass. Once that is done, the misexpect metadata can be updated to be smaller. In the long term, it is possible to reconstruct portions of the misexpect metadata from the existing profile data. However, we have avoided this to keep the code simple, and because some kind of metadata tag will be required to identify which branch/switch/select instructions are influenced by the use of llvm.expect Patch By: paulkirth Differential Revision: https://reviews.llvm.org/D66324 llvm-svn: 371635
42 lines
1.2 KiB
C
42 lines
1.2 KiB
C
// Test that misexpect detects mis-annotated switch statements
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// RUN: llvm-profdata merge %S/Inputs/misexpect-switch.proftext -o %t.profdata
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// RUN: %clang_cc1 %s -O2 -o - -emit-llvm -fprofile-instrument-use-path=%t.profdata -verify -Wmisexpect -debug-info-kind=line-tables-only
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int sum(int *buff, int size);
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int random_sample(int *buff, int size);
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int rand();
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void init_arry();
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const int inner_loop = 1000;
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const int outer_loop = 20;
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const int arry_size = 25;
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int arry[arry_size] = {0};
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int main() {
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init_arry();
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int val = 0;
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int j, k;
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for (j = 0; j < outer_loop; ++j) {
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for (k = 0; k < inner_loop; ++k) {
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unsigned condition = rand() % 10000;
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switch (__builtin_expect(condition, 0)) { // expected-warning-re {{Potential performance regression from use of __builtin_expect(): Annotation was correct on {{.+}}% ({{[0-9]+ / [0-9]+}}) of profiled executions.}}
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case 0:
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val += sum(arry, arry_size);
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break;
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case 1:
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case 2:
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case 3:
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break;
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default:
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val += random_sample(arry, arry_size);
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break;
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} // end switch
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} // end inner_loop
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} // end outer_loop
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return 0;
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}
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