[llvm-exegesis] Analysis: writeMeasurementValue(): don't alloc string for double each time.

Summary:
Test data: 500kLOC of benchmark.yaml, 23Mb. (that is a subset of the actual uops benchmark i was trying to analyze!)
Old time: (D54382)
```
 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=100000 -benchmarks-file=/tmp/benchmarks.yaml -analysis-inconsistencies-output-file=/tmp/clusters.html' (16 runs):

       9024.354355      task-clock (msec)         #    1.000 CPUs utilized            ( +-  0.18% )
...
            9.0262 +- 0.0161 seconds time elapsed  ( +-  0.18% )
```
New time:
```
 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=100000 -benchmarks-file=/tmp/benchmarks.yaml -analysis-inconsistencies-output-file=/tmp/clusters.html' (16 runs):

       8996.541057      task-clock (msec)         #    0.999 CPUs utilized            ( +-  0.19% )
...
            9.0045 +- 0.0172 seconds time elapsed  ( +-  0.19% )
```
-~0.3%, not that much. But this isn't the important part.

Old:
* calls to allocation functions: 2109712
* temporary allocations: 33112
* bytes allocated in total (ignoring deallocations): 4.43 GB

New:
* calls to allocation functions: 2095345 (-0.68%)
* temporary allocations: 18745 (-43.39% !!!)
* bytes allocated in total (ignoring deallocations): 4.31 GB (-2.71%)

Reviewers: courbet, MaskRay, RKSimon, gchatelet, john.brawn

Reviewed By: courbet

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D54383

llvm-svn: 347199

llvm/tools/llvm-exegesis/lib/Analysis.cpp

@@ -12,6 +12,7 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/Support/FormatVariadic.h"
+#include <limits>
 #include <unordered_set>
 #include <vector>
 
@@ -95,7 +96,21 @@ writeClusterId(llvm::raw_ostream &OS,
 
 template <EscapeTag Tag>
 static void writeMeasurementValue(llvm::raw_ostream &OS, const double Value) {
-  writeEscaped<Tag>(OS, llvm::formatv("{0:F}", Value).str());
+  // Given Value, if we wanted to serialize it to a string,
+  // how many base-10 digits will we need to store, max?
+  static constexpr auto MaxDigitCount =
+      std::numeric_limits<decltype(Value)>::max_digits10;
+  // Also, we will need a decimal separator.
+  static constexpr auto DecimalSeparatorLen = 1; // '.' e.g.
+  // So how long of a string will the serialization produce, max?
+  static constexpr auto SerializationLen = MaxDigitCount + DecimalSeparatorLen;
+
+  // WARNING: when changing the format, also adjust the small-size estimate ^.
+  static constexpr StringLiteral SimpleFloatFormat = StringLiteral("{0:F}");
+
+  writeEscaped<Tag>(
+      OS,
+      llvm::formatv(SimpleFloatFormat.data(), Value).sstr<SerializationLen>());
 }
 
 template <typename EscapeTag, EscapeTag Tag>