[MC] Use StringTable to reduce dynamic relocations (#144202)

Dynamic relocations are expensive on ELF/Linux platforms because they
are applied in userspace on process startup. Therefore, it is worth
optimizing them to make PIE and PIC dylib builds faster. In +asserts
builds (non-NDEBUG), nikic identified these schedule class name string
pointers as the leading source of dynamic relocations. [1]

This change uses llvm::StringTable and the StringToOffsetTable TableGen
helper to turn the string pointers into 32-bit offsets into a separate
character array.

The number of dynamic relocations is reduced by ~60%:
❯ llvm-readelf --dyn-relocations lib/libLLVM.so | wc -l
381376 # before
155156 # after

The test suite time is modestly affected, but I'm running on a shared
noisy workstation VM with a ton of cores:
https://gist.github.com/rnk/f38882c2fe2e63d0eb58b8fffeab69de
Testing Time: 100.88s   # before
Testing Time: 78.50s. # after
Testing Time: 96.25s.  # before again

I haven't used any fancy hyperfine/denoising tools, but I think the
result is clearly visible and we should ship it.

[1] https://gist.github.com/nikic/554f0a544ca15d5219788f1030f78c5a

1 files changed, 5 insertions, 4 deletions

diff --git a/llvm/tools/llvm-exegesis/lib/Analysis.cpp b/llvm/tools/llvm-exegesis/lib/Analysis.cpp
index be10c32..fb84328 100644
--- a/llvm/tools/llvm-exegesis/lib/Analysis.cpp
+++ b/llvm/tools/llvm-exegesis/lib/Analysis.cpp
@@ -137,9 +137,9 @@ void Analysis::printInstructionRowCsv(const size_t PointId,
   std::tie(SchedClassId, std::ignore) = ResolvedSchedClass::resolveSchedClassId(
       State_.getSubtargetInfo(), State_.getInstrInfo(), MCI);
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
-  const MCSchedClassDesc *const SCDesc =
-      State_.getSubtargetInfo().getSchedModel().getSchedClassDesc(SchedClassId);
-  writeEscaped<kEscapeCsv>(OS, SCDesc->Name);
+  StringRef SCDescName =
+      State_.getSubtargetInfo().getSchedModel().getSchedClassName(SchedClassId);
+  writeEscaped<kEscapeCsv>(OS, SCDescName);
 #else
   OS << SchedClassId;
 #endif
@@ -563,7 +563,8 @@ Error Analysis::run<Analysis::PrintSchedClassInconsistencies>(
     OS << "<div class=\"inconsistency\"><p>Sched Class <span "
           "class=\"sched-class-name\">";
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
-    writeEscaped<kEscapeHtml>(OS, RSCAndPoints.RSC.SCDesc->Name);
+    writeEscaped<kEscapeHtml>(OS, SI.getSchedModel().getSchedClassName(
+                                      RSCAndPoints.RSC.SchedClassId));
 #else
     OS << RSCAndPoints.RSC.SchedClassId;
 #endif