Avoid String::printf on a hot path.

Generating unique names is expensive! The inliner does it a lot. There
are two pain points:
- Assemble a candidate name with String::printf
- Use SymbolTable::lookup to make sure that name isn't taken already

Previously, these steps took roughly the same amount of time on my
machine. Afterwards, SymbolTable::lookup takes the vast majority of the
time spent in uniqueName. (Further optimizing symbol lookup is a Hard
Problem--trying to design something better than the current
implementation in the general case is not easy.)

This change gives nanobench a 1-3% win on my Macbook Pro.

median	mean
192µs	196µs	sksl_medium (before)
185µs	189µs	sksl_medium (after)

414µs	424µs	sksl_large  (before)
406µs	413µs	sksl_large  (after)

Change-Id: I8e0f7f053d124827a4d50a5006129e541a20c70d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/534520
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>

src/sksl/BUILD.bazel

@@ -641,8 +641,8 @@ generated_cc_atom(
     visibility = ["//:__subpackages__"],
     deps = [
         ":SkSLMangler_hdr",
+        "//include/core:SkString_hdr",
         "//include/core:SkTypes_hdr",
-        "//include/private:SkSLString_hdr",
         "//include/private:SkStringView_hdr",
         "//src/sksl/ir:SkSLSymbolTable_hdr",
     ],

src/sksl/SkSLMangler.cpp

@@ -7,11 +7,12 @@
 
 #include "src/sksl/SkSLMangler.h"
 
+#include "include/core/SkString.h"
 #include "include/core/SkTypes.h"
-#include "include/private/SkSLString.h"
 #include "include/private/SkStringView.h"
 #include "src/sksl/ir/SkSLSymbolTable.h"
 
+#include <algorithm>
 #include <ctype.h>
 
 namespace SkSL {
@@ -47,16 +48,28 @@ std::string Mangler::uniqueName(std::string_view baseName, SymbolTable* symbolTa
     // Append a unique numeric prefix to avoid name overlap. Check the symbol table to make sure
     // we're not reusing an existing name. (Note that within a single compilation pass, this check
     // isn't fully comprehensive, as code isn't always generated in top-to-bottom order.)
-    std::string uniqueName;
-    for (;;) {
-        uniqueName = SkSL::String::printf("_%d_%.*s", fCounter++,
-                                          (int)baseName.size(), baseName.data());
-        if ((*symbolTable)[uniqueName] == nullptr) {
-            break;
-        }
-    }
 
-    return uniqueName;
+    // This code is a performance hotspot. Assemble the string manually to save a few cycles.
+    char uniqueName[256];
+    uniqueName[0] = '_';
+    char* uniqueNameEnd = uniqueName + SK_ARRAY_COUNT(uniqueName);
+    for (;;) {
+        // _123
+        char* endPtr = SkStrAppendS32(uniqueName + 1, fCounter++);
+
+        // _123_
+        *endPtr++ = '_';
+
+        // _123_baseNameTruncatedToFit (no null terminator, because string_view doesn't require one)
+        int baseNameCopyLength = std::min<int>(baseName.size(), uniqueNameEnd - endPtr);
+        memcpy(endPtr, baseName.data(), baseNameCopyLength);
+        endPtr += baseNameCopyLength;
+
+        std::string_view uniqueNameView(uniqueName, endPtr - uniqueName);
+        if ((*symbolTable)[uniqueNameView] == nullptr) {
+            return std::string(uniqueNameView);
+        }
+    }
 }
 
 } // namespace SkSL