[llvm-branch-commits] [compiler-rt] 97260ab - [llvm-cov][gcov] Optimize the cycle counting algorithm by skipping zero count cycles
Fangrui Song via llvm-branch-commits
llvm-branch-commits at lists.llvm.org
Thu Dec 10 15:27:13 PST 2020
Author: Xinhao Yuan
Date: 2020-12-10T15:22:29-08:00
New Revision: 97260ab4786f87211b8553b56fd0600016b1d6fa
URL: https://github.com/llvm/llvm-project/commit/97260ab4786f87211b8553b56fd0600016b1d6fa
DIFF: https://github.com/llvm/llvm-project/commit/97260ab4786f87211b8553b56fd0600016b1d6fa.diff
LOG: [llvm-cov][gcov] Optimize the cycle counting algorithm by skipping zero count cycles
This change is similar to http://gcc.gnu.org/PR90380
This reduces the complexity from exponential to polynomial of the arcs.
Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D93036
Added:
compiler-rt/test/profile/gcov-complex-line.c
Modified:
llvm/include/llvm/ProfileData/GCOV.h
llvm/lib/ProfileData/GCOV.cpp
Removed:
################################################################################
diff --git a/compiler-rt/test/profile/gcov-complex-line.c b/compiler-rt/test/profile/gcov-complex-line.c
new file mode 100644
index 000000000000..86286c4a93a5
--- /dev/null
+++ b/compiler-rt/test/profile/gcov-complex-line.c
@@ -0,0 +1,55 @@
+// This test checks that the cycle detection algorithm in llvm-cov is able to
+// handle complex block graphs by skipping zero count cycles.
+//
+// RUN: mkdir -p %t.dir && cd %t.dir
+// RUN: %clang --coverage %s -o %t
+// RUN: rm -f gcov-complex-line.gcda && %run %t
+// RUN: llvm-cov gcov -t gcov-complex-line.c | FileCheck %s
+
+#define M_0 \
+ do { \
+ if (x == 0) \
+ x = 0; \
+ else \
+ x = 1; \
+ } while (0)
+#define M_1 \
+ do { \
+ M_0; \
+ M_0; \
+ M_0; \
+ M_0; \
+ } while (0)
+#define M_2 \
+ do { \
+ M_1; \
+ M_1; \
+ M_1; \
+ M_1; \
+ } while (0)
+#define M_3 \
+ do { \
+ M_2; \
+ M_2; \
+ M_2; \
+ M_2; \
+ } while (0)
+#define M_4 \
+ do { \
+ M_3; \
+ M_3; \
+ M_3; \
+ M_3; \
+ } while (0)
+#define COMPLEX_LINE \
+ do { \
+ for (int i = 0; i < 100; ++i) \
+ M_4; \
+ } while (0)
+
+int main() {
+ volatile int x = 0;
+ // In the following line, the number of cycles in the block graph is at least
+ // 2^256, where 256 is the number of expansions of M_0.
+ COMPLEX_LINE; // CHECK: 101: [[#@LINE]]: COMPLEX_LINE;
+}
diff --git a/llvm/include/llvm/ProfileData/GCOV.h b/llvm/include/llvm/ProfileData/GCOV.h
index 2766ff52e4a0..b15c5a6a6c65 100644
--- a/llvm/include/llvm/ProfileData/GCOV.h
+++ b/llvm/include/llvm/ProfileData/GCOV.h
@@ -295,6 +295,7 @@ class GCOVBlock {
static uint64_t getCycleCount(const Edges &Path);
static void unblock(const GCOVBlock *U, BlockVector &Blocked,
BlockVectorLists &BlockLists);
+ static void trimZeroCountSuffix(Edges &Path);
static bool lookForCircuit(const GCOVBlock *V, const GCOVBlock *Start,
Edges &Path, BlockVector &Blocked,
BlockVectorLists &BlockLists,
diff --git a/llvm/lib/ProfileData/GCOV.cpp b/llvm/lib/ProfileData/GCOV.cpp
index c0735250cfd8..b8bc0cc48376 100644
--- a/llvm/lib/ProfileData/GCOV.cpp
+++ b/llvm/lib/ProfileData/GCOV.cpp
@@ -427,6 +427,11 @@ LLVM_DUMP_METHOD void GCOVBlock::dump() const { print(dbgs()); }
// The algorithm in GCC is based on the algorithm by Hawick & James:
// "Enumerating Circuits and Loops in Graphs with Self-Arcs and Multiple-Arcs"
// http://complexity.massey.ac.nz/cstn/013/cstn-013.pdf.
+//
+// An optimization is to skip any arc with zero count and to backtrack if the
+// current path has such arcs: any cycle with such arc makes no contribution to
+// the final cycle count. This reduces the complexity from exponential to
+// polynomial of the arcs.
/// Get the count for the detected cycle.
uint64_t GCOVBlock::getCycleCount(const Edges &Path) {
@@ -458,6 +463,15 @@ void GCOVBlock::unblock(const GCOVBlock *U, BlockVector &Blocked,
}
}
+void GCOVBlock::trimZeroCountSuffix(Edges &Path) {
+ for (size_t index = 0; index < Path.size(); ++index) {
+ if (Path[index]->cycleCount == 0) {
+ Path.resize(index);
+ return;
+ }
+ }
+}
+
bool GCOVBlock::lookForCircuit(const GCOVBlock *V, const GCOVBlock *Start,
Edges &Path, BlockVector &Blocked,
BlockVectorLists &BlockLists,
@@ -465,10 +479,11 @@ bool GCOVBlock::lookForCircuit(const GCOVBlock *V, const GCOVBlock *Start,
Blocked.push_back(V);
BlockLists.emplace_back(BlockVector());
bool FoundCircuit = false;
+ const size_t PrefixLength = Path.size();
for (auto E : V->dsts()) {
const GCOVBlock *W = &E->dst;
- if (W < Start || find(Blocks, W) == Blocks.end()) {
+ if (E->cycleCount == 0 || W < Start || find(Blocks, W) == Blocks.end()) {
continue;
}
@@ -477,6 +492,7 @@ bool GCOVBlock::lookForCircuit(const GCOVBlock *V, const GCOVBlock *Start,
if (W == Start) {
// We've a cycle.
Count += GCOVBlock::getCycleCount(Path);
+ trimZeroCountSuffix(Path);
FoundCircuit = true;
} else if (find(Blocked, W) == Blocked.end() && // W is not blocked.
GCOVBlock::lookForCircuit(W, Start, Path, Blocked, BlockLists,
@@ -484,7 +500,10 @@ bool GCOVBlock::lookForCircuit(const GCOVBlock *V, const GCOVBlock *Start,
FoundCircuit = true;
}
- Path.pop_back();
+ if (Path.size() > PrefixLength)
+ Path.pop_back();
+ else if (Path.size() < PrefixLength)
+ break;
}
if (FoundCircuit) {
@@ -492,7 +511,7 @@ bool GCOVBlock::lookForCircuit(const GCOVBlock *V, const GCOVBlock *Start,
} else {
for (auto E : V->dsts()) {
const GCOVBlock *W = &E->dst;
- if (W < Start || find(Blocks, W) == Blocks.end()) {
+ if (E->cycleCount == 0 || W < Start || find(Blocks, W) == Blocks.end()) {
continue;
}
const size_t index = find(Blocked, W) - Blocked.begin();
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