[hwasan][NFCI] Rename ClRandomSkipRate to ClRandomKeepRate (llvm#126990)

The meaning of ClRandomSkipRate was inverted in
llvm#88070 but the variable name
was not changed. This patch fixes it to avoid confusion.

Additionally, it elaborates the flag description to mention the
interaction between the random keep rate and hotness cutoff.

Author: thurstond

llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp

@@ -195,9 +195,12 @@ static cl::opt<int> ClHotPercentileCutoff("hwasan-percentile-cutoff-hot",
                                           cl::desc("Hot percentile cutoff."));
 
 static cl::opt<float>
-    ClRandomSkipRate("hwasan-random-rate",
+    ClRandomKeepRate("hwasan-random-rate",
                      cl::desc("Probability value in the range [0.0, 1.0] "
-                              "to keep instrumentation of a function."));
+                              "to keep instrumentation of a function. "
+                              "Note: instrumentation can be skipped randomly "
+                              "OR because of the hot percentile cutoff, if "
+                              "both are supplied."));
 
 STATISTIC(NumTotalFuncs, "Number of total funcs");
 STATISTIC(NumInstrumentedFuncs, "Number of instrumented funcs");
@@ -301,7 +304,7 @@ class HWAddressSanitizer {
       : M(M), SSI(SSI) {
     this->Recover = optOr(ClRecover, Recover);
     this->CompileKernel = optOr(ClEnableKhwasan, CompileKernel);
-    this->Rng = ClRandomSkipRate.getNumOccurrences() ? M.createRNG(DEBUG_TYPE)
+    this->Rng = ClRandomKeepRate.getNumOccurrences() ? M.createRNG(DEBUG_TYPE)
                                                      : nullptr;
 
     initializeModule();
@@ -1599,9 +1602,9 @@ bool HWAddressSanitizer::selectiveInstrumentationShouldSkip(
   };
 
   auto SkipRandom = [&]() {
-    if (!ClRandomSkipRate.getNumOccurrences())
+    if (!ClRandomKeepRate.getNumOccurrences())
       return false;
-    std::bernoulli_distribution D(ClRandomSkipRate);
+    std::bernoulli_distribution D(ClRandomKeepRate);
     return !D(*Rng);
   };