Optimize Memory::ReadString
This function is responsible for majority of CPU time in prefetto.
Reduce the number of memory reads (don't read strings byte-by-byte).
Update all calls of ReadString to include the third parameter to have
a max read.
Add an Elf creation benchmark since this function is on the elf
creation path.
Test: libunwindstack_unit_test
Change-Id: Ia36e1f1a5ba76c9e9f13c43fb9e3691dde7897f2
diff --git a/libunwindstack/Memory.cpp b/libunwindstack/Memory.cpp
index 8de3d98..e142b97 100644
--- a/libunwindstack/Memory.cpp
+++ b/libunwindstack/Memory.cpp
@@ -158,20 +158,30 @@
return rc == size;
}
-bool Memory::ReadString(uint64_t addr, std::string* string, uint64_t max_read) {
- string->clear();
- uint64_t bytes_read = 0;
- while (bytes_read < max_read) {
- uint8_t value;
- if (!ReadFully(addr, &value, sizeof(value))) {
- return false;
+bool Memory::ReadString(uint64_t addr, std::string* dst, size_t max_read) {
+ char buffer[256]; // Large enough for 99% of symbol names.
+ size_t size = 0; // Number of bytes which were read into the buffer.
+ for (size_t offset = 0; offset < max_read; offset += size) {
+ // Look for null-terminator first, so we can allocate string of exact size.
+ // If we know the end of valid memory range, do the reads in larger blocks.
+ size_t read = std::min(sizeof(buffer), max_read - offset);
+ size = Read(addr + offset, buffer, read);
+ if (size == 0) {
+ return false; // We have not found end of string yet and we can not read more data.
}
- if (value == '\0') {
- return true;
+ size_t length = strnlen(buffer, size); // Index of the null-terminator.
+ if (length < size) {
+ // We found the null-terminator. Allocate the string and set its content.
+ if (offset == 0) {
+ // We did just single read, so the buffer already contains the whole string.
+ dst->assign(buffer, length);
+ return true;
+ } else {
+ // The buffer contains only the last block. Read the whole string again.
+ dst->assign(offset + length, '\0');
+ return ReadFully(addr, dst->data(), dst->size());
+ }
}
- string->push_back(value);
- addr++;
- bytes_read++;
}
return false;
}