[llvm-exegesis] Analysis::writeSnippet(): be smarter about memory allocations.

[lebedev.ri]

Summary

Test data: 500kLOC of benchmark.yaml, 23Mb. (that is a subset of the actual uops benchmark i was trying to analyze!)
Old time: (D54381)

$ time ./bin/llvm-exegesis -mode=analysis -analysis-epsilon=100000 -benchmarks-file=/tmp/benchmarks.yaml -analysis-inconsistencies-output-file=/tmp/clusters.html &> /dev/null

real    0m10.487s
user    0m9.745s
sys     0m0.740s

New time:

$ time ./bin/llvm-exegesis -mode=analysis -analysis-epsilon=100000 -benchmarks-file=/tmp/benchmarks.yaml -analysis-inconsistencies-output-file=/tmp/clusters.html &> /dev/null

real    0m9.599s
user    0m8.824s
sys     0m0.772s

Not that much, around -9%. But that is not the good part yet, again.

Old:

• calls to allocation functions: 3347676
• temporary allocations: 277818
• bytes allocated in total (ignoring deallocations): 10.52 GB

New:

• calls to allocation functions: 2109712 (-36%)
• temporary allocations: 33112 (-88%)
• bytes allocated in total (ignoring deallocations): 4.43 GB (-58% *sic*)

[MaskRay]

// FIXME: magic number.

The owner should give you a reasonable capacity :)

[lebedev.ri]

In D54382#1294788, @MaskRay wrote:

// FIXME: magic number.

The owner should give you a reasonable capacity :)

I will double-check later, but i think it is sufficient to avoid allocations here
for any single instruction (including it's operands).
(i.e. InstPrinterStr shouldn't ever need to be heap-allocated.)

[courbet]

In D54382#1294804, @lebedev.ri wrote:

In D54382#1294788, @MaskRay wrote:

// FIXME: magic number.

The owner should give you a reasonable capacity :)

I will double-check later, but i think it is sufficient to avoid allocations here
for any single instruction (including it's operands).
(i.e. InstPrinterStr shouldn't ever need to be heap-allocated.)

Yep, SGTM.

[lebedev.ri]

In D54382#1294788, @MaskRay wrote:

...

In D54382#1302604, @courbet wrote:

Yep, SGTM.

Thank you for the reviews!

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

while (!Bytes.empty()) {
     uint64_t MISize = 0;
     if (!Disasm_->getInstruction(MI, MISize, Bytes, 0, llvm::nulls(),
                                  llvm::nulls())) {
       writeEscaped<Tag>(OS, llvm::join(Lines, Separator));
       writeEscaped<Tag>(OS, Separator);
       writeEscaped<Tag>(OS, "[error decoding asm snippet]");
       return;
     }
-    Lines.emplace_back();
-    std::string &Line = Lines.back();
-    llvm::raw_string_ostream OSS(Line);
+    llvm::SmallString<128> InstPrinterStr; // FIXME: magic number.
+    llvm::raw_svector_ostream OSS(InstPrinterStr);
     InstPrinter_->printInst(&MI, OSS, "", *SubtargetInfo_);
     Bytes = Bytes.drop_front(MISize);
-    OSS.flush();
-    Line = llvm::StringRef(Line).trim().str();
+    Lines.emplace_back(llvm::StringRef(InstPrinterStr).trim());
   }
   writeEscaped<Tag>(OS, llvm::join(Lines, Separator));
 }
 
 // Prints a row representing an instruction, along with scheduling info and
 // point coordinates (measurements).
 void Analysis::printInstructionRowCsv(const size_t PointId,
                                       llvm::raw_ostream &OS) const {