[llvm-profgen] Add --sample-period to estimate absolute counts

llvm/test/tools/llvm-profgen/period-scaling.test

@@ -0,0 +1,84 @@
+// RUN: llvm-profgen --format=text --perfscript=%S/Inputs/cmov_3.perfscript --binary=%S/Inputs/cmov_3.perfbin --output=%t --skip-symbolization --perf-event=br_inst_retired.near_taken:upp --sample-period=1000003
+// RUN: FileCheck %s --input-file %t --check-prefix=CHECK-RAW-PROFILE
+// RUN: llvm-profgen --format=text --perfscript=%S/Inputs/cmov_3.perfscript --binary=%S/Inputs/cmov_3.perfbin --output=%t --perf-event=br_inst_retired.near_taken:upp --sample-period=1000003
+// RUN: FileCheck %s --input-file %t --check-prefix=CHECK
+
+// RUN: llvm-profgen --format=text --perfscript=%S/Inputs/cmov_3.perfscript --binary=%S/Inputs/cmov_3.perfbin --output=%t --skip-symbolization --perf-event=br_misp_retired.all_branches:upp --leading-ip-only --sample-period=1000003
+// RUN: FileCheck %s --input-file %t --check-prefix=UNPRED-RAW-PROFILE
+// RUN: llvm-profgen --format=text --perfscript=%S/Inputs/cmov_3.perfscript --binary=%S/Inputs/cmov_3.perfbin --output=%t --perf-event=br_misp_retired.all_branches:upp --leading-ip-only --sample-period=1000003
+// RUN: FileCheck %s --input-file %t --check-prefix=UNPRED
+
+// Check that we can use perf event filtering to generate multiple types of
+// source-level profiles from a single perf profile. In this case, we generate
+// a typical execution frequency profile using br_inst_retired.near_taken LBRs,
+// and a branch mispredict profile using br_misp_retired.all_branches sample
+// IPs.
+
+// Check that we can use --sample-period to compute LBR and IP-based profiles
+// which have comparable and absolute magnitudes. For example, in this case the
+// branch of interest (at source line offset 4) is in a loop body which is
+// executed ~20M times in total, and it's mispredicted about 9M times, yielding
+// a mispredict rate of roughly 0.45.
+
+// The source example below is based on perfKernelCpp/cmov_3, except a
+// misleading builtin is used to persuade the compiler not to use cmov, which
+// induces branch mispredicts.
+
+// CHECK: sel_arr:652547082:0
+// CHECK:  3.1: 20225766
+// CHECK:  3.2: 20225766
+// CHECK:  4: 19838670
+// CHECK:  5: 20225766
+
+// UNPRED: sel_arr:18000054:0
+// UNPRED:  3.1: 0
+// UNPRED:  3.2: 0
+// UNPRED:  4: 9000027
+// UNPRED:  5: 0
+
+// CHECK-RAW-PROFILE:      3
+// CHECK-RAW-PROFILE-NEXT: 2f0-2fa:9774174
+// CHECK-RAW-PROFILE-NEXT: 2f0-310:10064496
+// CHECK-RAW-PROFILE-NEXT: 2ff-310:10161270
+
+// UNPRED-RAW-PROFILE:      1
+// UNPRED-RAW-PROFILE-NEXT: 2fa-2fa:9000027
+
+// original code:
+// icx -fprofile-sample-generate lit.c
+#include <stdlib.h>
+
+#define N 20000
+#define ITERS 10000
+
+static int *m_s1, *m_s2, *m_s3, *m_dst;
+
+void init(void) {
+    m_s1 = malloc(sizeof(int)*N);
+    m_s2 = malloc(sizeof(int)*N);
+    m_s3 = malloc(sizeof(int)*N);
+    m_dst = malloc(sizeof(int)*N);
+    srand(42);
+
+    for (int i = 0; i < N; i++) {
+        m_s1[i] = rand() % N;
+        m_s2[i] = 0;
+        m_s3[i] = 1;
+    }
+}
+
+void __attribute__((noinline)) sel_arr(int *dst, int *s1, int *s2, int *s3) {
+#pragma nounroll
+#pragma clang loop vectorize(disable) interleave(disable)
+    for (int i = 0; i < N; i++) {
+        int *p = __builtin_expect((s1[i] < 10035), 0) ? &s2[i] : &s3[i];
+        dst[i] = *p;
+    }
+}
+
+int main(void) {
+  init();
+  for(int i=0; i<ITERS; ++i)
+    sel_arr(m_dst, m_s1, m_s2, m_s3);
+  return 0;
+}

llvm/tools/llvm-profgen/PerfReader.cpp

@@ -53,6 +53,10 @@ static cl::alias
                           cl::desc("Comma-delimited version of -perf-event"),
                           cl::aliasopt(PerfEventFilter));
 
+static cl::opt<uint64_t>
+    SamplePeriod("sample-period", cl::init(1),
+                 cl::desc("The sampling period (-c) used for perf data"));
+
 extern cl::opt<std::string> PerfTraceFilename;
 extern cl::opt<bool> ShowDisassemblyOnly;
 extern cl::opt<bool> ShowSourceLocations;
@@ -1000,6 +1004,16 @@ void LBRPerfReader::parseSample(TraceStream &TraceIt, uint64_t Count) {
   if (extractLBRStack(TraceIt, Sample->LBRStack)) {
     warnIfMissingMMap();
     // Record LBR only samples by aggregation
+    // If a sampling period is given we can adjust the magnitude of sample
+    // counts to estimate the absolute magnitute.
+    if (SamplePeriod.getNumOccurrences()) {
+      Count *= SamplePeriod;
+      // If counts are LBR-based, as opposed to IP-based, then the magnitude is
+      // now amplified by roughly the LBR stack size. By adjusting this down, we
+      // can produce LBR-based and IP-based profiles with comparable magnitudes.
+      if (!LeadingIPOnly && Sample->LBRStack.size() > 1)
+        Count /= (Sample->LBRStack.size() - 1);
+    }
     AggregatedSamples[Hashable<PerfSample>(Sample)] += Count;
   }
 }