[llvm] [XRay] Reserve memory space ahead-of-time when reading native format log (PR #76853)

[Min-Yih Hsu via llvm-commits]

https://github.com/mshockwave created https://github.com/llvm/llvm-project/pull/76853

XRay used to struggle reading large log files. It turned out the bottleneck was primarily caused by the reallocation happens when appending log entries into a std::vector.
This patch reserves the memory space ahead-of-time since the number of entries is known for most cases. Making llvm-xray runs 1.8 times faster and uses 1.4 times less physical memory when reading large (~2.6GB) log files.

-------
Here are the benchmark numbers (benchmarking using hyperfine):
```
Benchmark 1: llvm-xray account --instr_map=../prog ./xray-log.prog.DnIfOu
  Time (mean ± σ):     20.606 s ±  1.127 s    [User: 9.467 s, System: 11.081 s]
  Range (min … max):   20.051 s … 23.296 s    10 runs

Benchmark 2: env XRAY_RESERVE_MEM=1 llvm-xray account --instr_map=../prog ./xray-log. prog.DnIfOu
  Time (mean ± σ):     11.238 s ±  0.017 s    [User: 6.544 s, System: 4.664 s]
  Range (min … max):   11.207 s … 11.261 s    10 runs

Summary
  env XRAY_RESERVE_MEM=1 llvm-xray account --instr_map=../prog ./xray-log.prog.DnIfOu ran
    1.83 ± 0.10 times faster than llvm-xray account --instr_map=../prog ./xray-log.prog.DnIfOu
```
Notes:
1. The XRay mode used here is `xray-basic`
2. `xray-log.prog.DnIfOu` is around 2.6GB
3. `XRAY_RESERVE_MEM` is a temporary environment variable solely for benchmarking purposes. We are NOT using it to toggle the improvement in this patch

>From 5e5e5e8a8ff2ee5151a440a053f3102c73dcc1e4 Mon Sep 17 00:00:00 2001
From: Min Hsu <min.hsu at sifive.com>
Date: Wed, 3 Jan 2024 10:16:03 -0800
Subject: [PATCH] [XRay] Reserve memory space ahead-of-time when reading native
 format log

XRay used to struggle reading large log files. It turned out the
bottleneck was primarily caused by the reallocation happens when
appending log entries into a std::vector.
This patch reserves the memory space ahead-of-time since the number of
entries is known for most cases. Making llvm-xray runs 1.8 times faster
and uses 1.4 times less physical memory when reading large (~2.6GB)
log files.
---
 llvm/lib/XRay/Trace.cpp | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/llvm/lib/XRay/Trace.cpp b/llvm/lib/XRay/Trace.cpp
index b870adf565459f..74515b16417da3 100644
--- a/llvm/lib/XRay/Trace.cpp
+++ b/llvm/lib/XRay/Trace.cpp
@@ -51,6 +51,9 @@ Error loadNaiveFormatLog(StringRef Data, bool IsLittleEndian,
     return FileHeaderOrError.takeError();
   FileHeader = std::move(FileHeaderOrError.get());
 
+  size_t NumReservations = llvm::divideCeil(Reader.size() - OffsetPtr, 32U);
+  Records.reserve(NumReservations);
+
   // Each record after the header will be 32 bytes, in the following format:
   //
   //   (2)   uint16 : record type