Speed-up absl::Symbolize by ~6x via faster file reads.

absl::Symbolize does tons of tiny reads. Speed this up by switching
from lseek+read to a pread, and by reading more data than requested
into a buffer.

A faster absl::Symbolize will be helpful in tests and when printing
stack traces on /threadz etc.

Results for absl::Symbolize benchmark that exercises uncached behavior
of absl::Symbolize:

```
name          old time/op  new time/op  delta
BM_Symbolize  16.4ms ±12%   2.6ms ± 0%  -84.06%  (p=0.001 n=5+9)
```

PiperOrigin-RevId: 582687566
Change-Id: I44caf189d81867f3fd8c050a3100a4b9a8e744d7

1 files changed, 108 insertions, 44 deletions

diff --git a/absl/debugging/symbolize_elf.inc b/absl/debugging/symbolize_elf.inc
index 30638cb2..ae75cd41 100644
--- a/absl/debugging/symbolize_elf.inc
+++ b/absl/debugging/symbolize_elf.inc
@@ -289,6 +289,30 @@ ObjFile *AddrMap::Add() {
   return new (&obj_[size_++]) ObjFile;
 }
 
+class CachingFile {
+ public:
+  // Setup reader for fd that uses buf[0, buf_size-1] as a cache.
+  CachingFile(int fd, char *buf, size_t buf_size)
+      : fd_(fd),
+        cache_(buf),
+        cache_size_(buf_size),
+        cache_start_(0),
+        cache_limit_(0) {}
+
+  int fd() const { return fd_; }
+  ssize_t ReadFromOffset(void *buf, size_t count, off_t offset);
+  bool ReadFromOffsetExact(void *buf, size_t count, off_t offset);
+
+ private:
+  // Bytes [cache_start_, cache_limit_-1] from fd_ are stored in
+  // a prefix of cache_[0, cache_size_-1].
+  int fd_;
+  char *cache_;
+  size_t cache_size_;
+  off_t cache_start_;
+  off_t cache_limit_;
+};
+
 // ---------------------------------------------------------------
 
 enum FindSymbolResult { SYMBOL_NOT_FOUND = 1, SYMBOL_TRUNCATED, SYMBOL_FOUND };
@@ -330,6 +354,7 @@ class Symbolizer {
     SYMBOL_BUF_SIZE = 3072,
     TMP_BUF_SIZE = 1024,
     SYMBOL_CACHE_LINES = 128,
+    FILE_CACHE_SIZE = 8192,
   };
 
   AddrMap addr_map_;
@@ -338,6 +363,7 @@ class Symbolizer {
   bool addr_map_read_;
 
   char symbol_buf_[SYMBOL_BUF_SIZE];
+  char file_cache_[FILE_CACHE_SIZE];
 
   // tmp_buf_ will be used to store arrays of ElfW(Shdr) and ElfW(Sym)
   // so we ensure that tmp_buf_ is properly aligned to store either.
@@ -436,34 +462,58 @@ static ssize_t ReadPersistent(int fd, void *buf, size_t count) {
   return static_cast<ssize_t>(num_bytes);
 }
 
-// Read up to "count" bytes from "offset" in the file pointed by file
-// descriptor "fd" into the buffer starting at "buf".  On success,
-// return the number of bytes read.  Otherwise, return -1.
-static ssize_t ReadFromOffset(const int fd, void *buf, const size_t count,
-                              const off_t offset) {
-  off_t off = lseek(fd, offset, SEEK_SET);
-  if (off == (off_t)-1) {
-    ABSL_RAW_LOG(WARNING, "lseek(%d, %jd, SEEK_SET) failed: errno=%d", fd,
-                 static_cast<intmax_t>(offset), errno);
-    return -1;
+// Read up to "count" bytes from "offset" into the buffer starting at "buf",
+// while handling short reads and EINTR.  On success, return the number of bytes
+// read.  Otherwise, return -1.
+ssize_t CachingFile::ReadFromOffset(void *buf, size_t count, off_t offset) {
+  char *dst = static_cast<char *>(buf);
+  size_t read = 0;
+  while (read < count) {
+    // Look in cache first.
+    if (offset >= cache_start_ && offset < cache_limit_) {
+      const char *hit_start = &cache_[offset - cache_start_];
+      const size_t n =
+          std::min(count - read, static_cast<size_t>(cache_limit_ - offset));
+      memcpy(dst, hit_start, n);
+      dst += n;
+      read += static_cast<size_t>(n);
+      offset += static_cast<off_t>(n);
+      continue;
+    }
+
+    cache_start_ = 0;
+    cache_limit_ = 0;
+    ssize_t n = pread(fd_, cache_, cache_size_, offset);
+    if (n < 0) {
+      if (errno == EINTR) {
+        continue;
+      }
+      ABSL_RAW_LOG(WARNING, "read failed: errno=%d", errno);
+      return -1;
+    }
+    if (n == 0) {  // Reached EOF.
+      break;
+    }
+
+    cache_start_ = offset;
+    cache_limit_ = offset + static_cast<off_t>(n);
+    // Next iteration will copy from cache into dst.
   }
-  return ReadPersistent(fd, buf, count);
+  return static_cast<ssize_t>(read);
 }
 
-// Try reading exactly "count" bytes from "offset" bytes in a file
-// pointed by "fd" into the buffer starting at "buf" while handling
-// short reads and EINTR.  On success, return true. Otherwise, return
-// false.
-static bool ReadFromOffsetExact(const int fd, void *buf, const size_t count,
-                                const off_t offset) {
-  ssize_t len = ReadFromOffset(fd, buf, count, offset);
+// Try reading exactly "count" bytes from "offset" bytes into the buffer
+// starting at "buf" while handling short reads and EINTR.  On success, return
+// true. Otherwise, return false.
+bool CachingFile::ReadFromOffsetExact(void *buf, size_t count, off_t offset) {
+  ssize_t len = ReadFromOffset(buf, count, offset);
   return len >= 0 && static_cast<size_t>(len) == count;
 }
 
 // Returns elf_header.e_type if the file pointed by fd is an ELF binary.
-static int FileGetElfType(const int fd) {
+static int FileGetElfType(CachingFile *file) {
   ElfW(Ehdr) elf_header;
-  if (!ReadFromOffsetExact(fd, &elf_header, sizeof(elf_header), 0)) {
+  if (!file->ReadFromOffsetExact(&elf_header, sizeof(elf_header), 0)) {
     return -1;
   }
   if (memcmp(elf_header.e_ident, ELFMAG, SELFMAG) != 0) {
@@ -478,8 +528,8 @@ static int FileGetElfType(const int fd) {
 // To keep stack consumption low, we would like this function to not get
 // inlined.
 static ABSL_ATTRIBUTE_NOINLINE bool GetSectionHeaderByType(
-    const int fd, ElfW(Half) sh_num, const off_t sh_offset, ElfW(Word) type,
-    ElfW(Shdr) * out, char *tmp_buf, size_t tmp_buf_size) {
+    CachingFile *file, ElfW(Half) sh_num, const off_t sh_offset,
+    ElfW(Word) type, ElfW(Shdr) * out, char *tmp_buf, size_t tmp_buf_size) {
   ElfW(Shdr) *buf = reinterpret_cast<ElfW(Shdr) *>(tmp_buf);
   const size_t buf_entries = tmp_buf_size / sizeof(buf[0]);
   const size_t buf_bytes = buf_entries * sizeof(buf[0]);
@@ -490,7 +540,7 @@ static ABSL_ATTRIBUTE_NOINLINE bool GetSectionHeaderByType(
     const size_t num_bytes_to_read =
         (buf_bytes > num_bytes_left) ? num_bytes_left : buf_bytes;
     const off_t offset = sh_offset + static_cast<off_t>(i * sizeof(buf[0]));
-    const ssize_t len = ReadFromOffset(fd, buf, num_bytes_to_read, offset);
+    const ssize_t len = file->ReadFromOffset(buf, num_bytes_to_read, offset);
     if (len < 0) {
       ABSL_RAW_LOG(
           WARNING,
@@ -524,11 +574,17 @@ static ABSL_ATTRIBUTE_NOINLINE bool GetSectionHeaderByType(
 // but there has (as yet) been no need for anything longer either.
 const int kMaxSectionNameLen = 64;
 
+// Small cache to use for miscellaneous file reads.
+const int kSmallFileCacheSize = 100;
+
 bool ForEachSection(int fd,
                     const std::function<bool(absl::string_view name,
                                              const ElfW(Shdr) &)> &callback) {
+  char buf[kSmallFileCacheSize];
+  CachingFile file(fd, buf, sizeof(buf));
+
   ElfW(Ehdr) elf_header;
-  if (!ReadFromOffsetExact(fd, &elf_header, sizeof(elf_header), 0)) {
+  if (!file.ReadFromOffsetExact(&elf_header, sizeof(elf_header), 0)) {
     return false;
   }
 
@@ -540,7 +596,7 @@ bool ForEachSection(int fd,
   ElfW(Shdr) shstrtab;
   off_t shstrtab_offset = static_cast<off_t>(elf_header.e_shoff) +
                           elf_header.e_shentsize * elf_header.e_shstrndx;
-  if (!ReadFromOffsetExact(fd, &shstrtab, sizeof(shstrtab), shstrtab_offset)) {
+  if (!file.ReadFromOffsetExact(&shstrtab, sizeof(shstrtab), shstrtab_offset)) {
     return false;
   }
 
@@ -548,13 +604,13 @@ bool ForEachSection(int fd,
     ElfW(Shdr) out;
     off_t section_header_offset =
         static_cast<off_t>(elf_header.e_shoff) + elf_header.e_shentsize * i;
-    if (!ReadFromOffsetExact(fd, &out, sizeof(out), section_header_offset)) {
+    if (!file.ReadFromOffsetExact(&out, sizeof(out), section_header_offset)) {
       return false;
     }
     off_t name_offset = static_cast<off_t>(shstrtab.sh_offset) + out.sh_name;
     char header_name[kMaxSectionNameLen];
     ssize_t n_read =
-        ReadFromOffset(fd, &header_name, kMaxSectionNameLen, name_offset);
+        file.ReadFromOffset(&header_name, kMaxSectionNameLen, name_offset);
     if (n_read < 0) {
       return false;
     } else if (n_read > kMaxSectionNameLen) {
@@ -584,8 +640,10 @@ bool GetSectionHeaderByName(int fd, const char *name, size_t name_len,
     return false;
   }
 
+  char buf[kSmallFileCacheSize];
+  CachingFile file(fd, buf, sizeof(buf));
   ElfW(Ehdr) elf_header;
-  if (!ReadFromOffsetExact(fd, &elf_header, sizeof(elf_header), 0)) {
+  if (!file.ReadFromOffsetExact(&elf_header, sizeof(elf_header), 0)) {
     return false;
   }
 
@@ -597,18 +655,18 @@ bool GetSectionHeaderByName(int fd, const char *name, size_t name_len,
   ElfW(Shdr) shstrtab;
   off_t shstrtab_offset = static_cast<off_t>(elf_header.e_shoff) +
                           elf_header.e_shentsize * elf_header.e_shstrndx;
-  if (!ReadFromOffsetExact(fd, &shstrtab, sizeof(shstrtab), shstrtab_offset)) {
+  if (!file.ReadFromOffsetExact(&shstrtab, sizeof(shstrtab), shstrtab_offset)) {
     return false;
   }
 
   for (int i = 0; i < elf_header.e_shnum; ++i) {
     off_t section_header_offset =
         static_cast<off_t>(elf_header.e_shoff) + elf_header.e_shentsize * i;
-    if (!ReadFromOffsetExact(fd, out, sizeof(*out), section_header_offset)) {
+    if (!file.ReadFromOffsetExact(out, sizeof(*out), section_header_offset)) {
       return false;
     }
     off_t name_offset = static_cast<off_t>(shstrtab.sh_offset) + out->sh_name;
-    ssize_t n_read = ReadFromOffset(fd, &header_name, name_len, name_offset);
+    ssize_t n_read = file.ReadFromOffset(&header_name, name_len, name_offset);
     if (n_read < 0) {
       return false;
     } else if (static_cast<size_t>(n_read) != name_len) {
@@ -683,7 +741,7 @@ static const char *ComputeOffset(const char *base, ptrdiff_t offset) {
 // To keep stack consumption low, we would like this function to not get
 // inlined.
 static ABSL_ATTRIBUTE_NOINLINE FindSymbolResult FindSymbol(
-    const void *const pc, const int fd, char *out, size_t out_size,
+    const void *const pc, CachingFile *file, char *out, size_t out_size,
     ptrdiff_t relocation, const ElfW(Shdr) * strtab, const ElfW(Shdr) * symtab,
     const ElfW(Shdr) * opd, char *tmp_buf, size_t tmp_buf_size) {
   if (symtab == nullptr) {
@@ -716,7 +774,7 @@ static ABSL_ATTRIBUTE_NOINLINE FindSymbolResult FindSymbol(
     const size_t entries_in_chunk =
         std::min(num_remaining_symbols, buf_entries);
     const size_t bytes_in_chunk = entries_in_chunk * sizeof(buf[0]);
-    const ssize_t len = ReadFromOffset(fd, buf, bytes_in_chunk, offset);
+    const ssize_t len = file->ReadFromOffset(buf, bytes_in_chunk, offset);
     SAFE_ASSERT(len >= 0);
     SAFE_ASSERT(static_cast<size_t>(len) % sizeof(buf[0]) == 0);
     const size_t num_symbols_in_buf = static_cast<size_t>(len) / sizeof(buf[0]);
@@ -772,12 +830,12 @@ static ABSL_ATTRIBUTE_NOINLINE FindSymbolResult FindSymbol(
   if (found_match) {
     const off_t off =
         static_cast<off_t>(strtab->sh_offset) + best_match.st_name;
-    const ssize_t n_read = ReadFromOffset(fd, out, out_size, off);
+    const ssize_t n_read = file->ReadFromOffset(out, out_size, off);
     if (n_read <= 0) {
       // This should never happen.
       ABSL_RAW_LOG(WARNING,
-                   "Unable to read from fd %d at offset %lld: n_read = %zd", fd,
-                   static_cast<long long>(off), n_read);
+                   "Unable to read from fd %d at offset %lld: n_read = %zd",
+                   file->fd(), static_cast<long long>(off), n_read);
       return SYMBOL_NOT_FOUND;
     }
     ABSL_RAW_CHECK(static_cast<size_t>(n_read) <= out_size,
@@ -827,22 +885,24 @@ FindSymbolResult Symbolizer::GetSymbolFromObjectFile(
     }
   }
 
+  CachingFile file(obj.fd, file_cache_, sizeof(file_cache_));
+
   // Consult a regular symbol table, then fall back to the dynamic symbol table.
   for (const auto symbol_table_type : {SHT_SYMTAB, SHT_DYNSYM}) {
-    if (!GetSectionHeaderByType(obj.fd, obj.elf_header.e_shnum,
+    if (!GetSectionHeaderByType(&file, obj.elf_header.e_shnum,
                                 static_cast<off_t>(obj.elf_header.e_shoff),
                                 static_cast<ElfW(Word)>(symbol_table_type),
                                 &symtab, tmp_buf, tmp_buf_size)) {
       continue;
     }
-    if (!ReadFromOffsetExact(
-            obj.fd, &strtab, sizeof(strtab),
+    if (!file.ReadFromOffsetExact(
+            &strtab, sizeof(strtab),
             static_cast<off_t>(obj.elf_header.e_shoff +
                                symtab.sh_link * sizeof(symtab)))) {
       continue;
     }
     const FindSymbolResult rc =
-        FindSymbol(pc, obj.fd, out, out_size, relocation, &strtab, &symtab,
+        FindSymbol(pc, &file, out, out_size, relocation, &strtab, &symtab,
                    opd_ptr, tmp_buf, tmp_buf_size);
     if (rc != SYMBOL_NOT_FOUND) {
       return rc;
@@ -1323,15 +1383,19 @@ static bool MaybeInitializeObjFile(ObjFile *obj) {
       ABSL_RAW_LOG(WARNING, "%s: open failed: errno=%d", obj->filename, errno);
       return false;
     }
-    obj->elf_type = FileGetElfType(obj->fd);
+
+    char buf[kSmallFileCacheSize];
+    CachingFile file(obj->fd, buf, sizeof(buf));
+
+    obj->elf_type = FileGetElfType(&file);
     if (obj->elf_type < 0) {
       ABSL_RAW_LOG(WARNING, "%s: wrong elf type: %d", obj->filename,
                    obj->elf_type);
       return false;
     }
 
-    if (!ReadFromOffsetExact(obj->fd, &obj->elf_header, sizeof(obj->elf_header),
-                             0)) {
+    if (!file.ReadFromOffsetExact(&obj->elf_header, sizeof(obj->elf_header),
+                                  0)) {
       ABSL_RAW_LOG(WARNING, "%s: failed to read elf header", obj->filename);
       return false;
     }
@@ -1341,7 +1405,7 @@ static bool MaybeInitializeObjFile(ObjFile *obj) {
     size_t num_interesting_load_segments = 0;
     for (int j = 0; j < phnum; j++) {
       ElfW(Phdr) phdr;
-      if (!ReadFromOffsetExact(obj->fd, &phdr, sizeof(phdr), phoff)) {
+      if (!file.ReadFromOffsetExact(&phdr, sizeof(phdr), phoff)) {
         ABSL_RAW_LOG(WARNING, "%s: failed to read program header %d",
                      obj->filename, j);
         return false;