CanonicalizePath: Remove kMaxComponents limit

This patch refactors the CanonicalizePath() to fix
two issues and improve performance. This is achieved
through the following:

- Remove the kMaxPathComponents limit entirely,
  which fixes ninja-build#1732, by dropping the
  `components` array entirely, in favor of
  back-tracking the destination pointer.

- Properly handle '/' and '\' which were incorrectly
  converted into an empty string. This fixes
  ninja-build#2008.

- Skip initial '../' components in relative paths,
  as these are common when referencing source files
  in build plans, and doing so make the loop after
  this step run faster in practice since most
  source files do not need adjustments.

- Simplify the inner loop logic by handling the
  last component (which is not followed by a
  trailing directory separator) separately.

  This noticeably improves performance because
  the inner loop becomes smaller with less branch
  mis-predictions in the general case.

- Never access or copy the caharacter after the
  end of the input string.

- Use memchr() to find the next '/' on Posix, which
  allows the use of SIMD implementations provided by
  the C runtime (e.g. through IFUNC functions on Linux),
  resulting in very noticeable speedup. This is also
  why a statically Ninja executable will be slower
  than one that links to the C library dynamically :-/

- Avoid performing any writes when the input path
  doesn't need any adjustment, which is also quite
  common.

Note that this patch does _not_ remove the 64-bit
limit for the `slash_bits`  value, which is only
used on Win32.

Benchmarking was done in several ways:

- On Linux, running `hyperfine canon-perftest` to run the
  canonicalization benchmark program and measure its total
  running time. Three compilers were used to generate
  dynamically-linked executables.

  ```
               BEFORE (ms)  AFTER (ms)

  GCC 13.2.0      651         369
  Clang 14.0      591         402
  Clang 18,0      653         400
  ```

- On Windows, running `canon-perftest` 5 times and keeping
  the best reported average result. The number are slower
  since they only measure the benched function.

  ```
                   BEFORE (ms)  AFTER (ms)

  Mingw64 GCC 12      246         195
  ```

- On Linux, run `hyperfine ninja -C out/default -n --quiet`
  on a large Fuchsia build plan, once with 70000+ pending
  commands, and once after the build (i.e. `ninja: no work to do`).

  ````
               BEFORE (s)  AFTER (s)

  pre_build       8.789    8.647
  post_build      6.703    6.590
  ```

2 files changed, 208 insertions, 42 deletions

diff --git a/src/util.cc b/src/util.cc
index 0553de3..5f67fcf 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -143,20 +143,19 @@ void CanonicalizePath(char* path, size_t* len, uint64_t* slash_bits) {
     return;
   }
 
-  const int kMaxPathComponents = 60;
-  char* components[kMaxPathComponents];
-  int component_count = 0;
-
   char* start = path;
   char* dst = start;
+  char* dst_start = dst;
   const char* src = start;
   const char* end = start + *len;
+  const char* src_next;
 
+  // For absolute paths, skip the leading directory separator
+  // as this one should never be removed from the result.
   if (IsPathSeparator(*src)) {
 #ifdef _WIN32
-
-    // network path starts with //
-    if (*len > 1 && IsPathSeparator(*(src + 1))) {
+    // Windows network path starts with //
+    if (src + 2 <= end && IsPathSeparator(src[1])) {
       src += 2;
       dst += 2;
     } else {
@@ -167,50 +166,126 @@ void CanonicalizePath(char* path, size_t* len, uint64_t* slash_bits) {
     ++src;
     ++dst;
 #endif
+    dst_start = dst;
+  } else {
+    // For relative paths, skip any leading ../ as these are quite common
+    // to reference source files in build plans, and doing this here makes
+    // the loop work below faster in general.
+    while (src + 3 <= end && src[0] == '.' && src[1] == '.' &&
+           IsPathSeparator(src[2])) {
+      src += 3;
+      dst += 3;
+    }
   }
 
-  while (src < end) {
-    if (*src == '.') {
-      if (src + 1 == end || IsPathSeparator(src[1])) {
-        // '.' component; eliminate.
-        src += 2;
-        continue;
-      } else if (src[1] == '.' && (src + 2 == end || IsPathSeparator(src[2]))) {
-        // '..' component.  Back up if possible.
+  // Loop over all components of the paths _except_ the last one, in
+  // order to simplify the loop's code and make it faster.
+  int component_count = 0;
+  char* dst0 = dst;
+  for (; src < end; src = src_next) {
+#ifndef _WIN32
+    // Use memchr() for faster lookups thanks to optimized C library
+    // implementation. `hyperfine canon_perftest` shows a significant
+    // difference (e,g, 484ms vs 437ms).
+    const char* next_sep =
+        static_cast<const char*>(::memchr(src, '/', end - src));
+    if (!next_sep) {
+      // This is the last component, will be handled out of the loop.
+      break;
+    }
+#else
+    // Need to check for both '/' and '\\' so do not use memchr().
+    // Cannot use strpbrk() because end[0] can be \0 or something else!
+    const char* next_sep = src;
+    while (next_sep != end && !IsPathSeparator(*next_sep))
+      ++next_sep;
+    if (next_sep == end) {
+      // This is the last component, will be handled out of the loop.
+      break;
+    }
+#endif
+    // Position for next loop iteration.
+    src_next = next_sep + 1;
+    // Length of the component, excluding trailing directory.
+    size_t component_len = next_sep - src;
+
+    if (component_len <= 2) {
+      if (component_len == 0) {
+        continue;  // Ignore empty component, e.g. 'foo//bar' -> 'foo/bar'.
+      }
+      if (src[0] == '.') {
+        if (component_len == 1) {
+          continue;  // Ignore '.' component, e.g. './foo' -> 'foo'.
+        } else if (src[1] == '.') {
+          // Process the '..' component if found. Back up if possible.
+          if (component_count > 0) {
+            // Move back to start of previous component.
+            --component_count;
+            while (--dst > dst0 && !IsPathSeparator(dst[-1])) {
+              // nothing to do here, decrement happens before condition check.
+            }
+          } else {
+            dst[0] = '.';
+            dst[1] = '.';
+            dst[2] = src[2];
+            dst += 3;
+          }
+          continue;
+        }
+      }
+    }
+    ++component_count;
+
+    // Copy or skip component, including trailing directory separator.
+    if (dst != src) {
+      ::memmove(dst, src, src_next - src);
+    }
+    dst += src_next - src;
+  }
+
+  // Handling the last component that does not have a trailing separator.
+  // The logic here is _slightly_ different since there is no trailing
+  // directory separator.
+  size_t component_len = end - src;
+  do {
+    if (component_len == 0)
+      break;  // Ignore empty component (e.g. 'foo//' -> 'foo/')
+    if (src[0] == '.') {
+      if (component_len == 1)
+        break;  // Ignore trailing '.' (e.g. 'foo/.' -> 'foo/')
+      if (src[1] == '.') {
+        // Handle '..'. Back up if possible.
         if (component_count > 0) {
-          dst = components[component_count - 1];
-          src += 3;
-          --component_count;
+          while (--dst > dst0 && !IsPathSeparator(dst[-1])) {
+            // nothing to do here, decrement happens before condition check.
+          }
         } else {
-          *dst++ = *src++;
-          *dst++ = *src++;
-          *dst++ = *src++;
+          dst[0] = '.';
+          dst[1] = '.';
+          dst += 2;
+          // No separator to add here.
         }
-        continue;
+        break;
       }
     }
-
-    if (IsPathSeparator(*src)) {
-      src++;
-      continue;
+    // Skip or copy last component, no trailing separator.
+    if (dst != src) {
+      ::memmove(dst, src, component_len);
     }
+    dst += component_len;
+  } while (0);
 
-    if (component_count == kMaxPathComponents)
-      Fatal("path has too many components : %s", path);
-    components[component_count] = dst;
-    ++component_count;
-
-    while (src != end && !IsPathSeparator(*src))
-      *dst++ = *src++;
-    *dst++ = *src++;  // Copy '/' or final \0 character as well.
-  }
+  // Remove trailing path separator if any, but keep the initial
+  // path separator(s) if there was one (or two on Windows).
+  if (dst > dst_start && IsPathSeparator(dst[-1]))
+    dst--;
 
   if (dst == start) {
+    // Handle special cases like "aa/.." -> "."
     *dst++ = '.';
-    *dst++ = '\0';
   }
 
-  *len = dst - start - 1;
+  *len = dst - start;  // dst points after the trailing char here.
 #ifdef _WIN32
   uint64_t bits = 0;
   uint64_t bits_mask = 1;
diff --git a/src/util_test.cc b/src/util_test.cc
index d58b170..8467e2a 100644
--- a/src/util_test.cc
+++ b/src/util_test.cc
@@ -89,13 +89,57 @@ TEST(CanonicalizePath, PathSamples) {
   EXPECT_EQ("/foo", path);
 #endif
 
+  path = "..";
+  CanonicalizePath(&path);
+  EXPECT_EQ("..", path);
+
+  path = "../";
+  CanonicalizePath(&path);
+  EXPECT_EQ("..", path);
+
+  path = "../foo";
+  CanonicalizePath(&path);
+  EXPECT_EQ("../foo", path);
+
+  path = "../foo/";
+  CanonicalizePath(&path);
+  EXPECT_EQ("../foo", path);
+
+  path = "../..";
+  CanonicalizePath(&path);
+  EXPECT_EQ("../..", path);
+
+  path = "../../";
+  CanonicalizePath(&path);
+  EXPECT_EQ("../..", path);
+
+  path = "./../";
+  CanonicalizePath(&path);
+  EXPECT_EQ("..", path);
+
+  path = "/..";
+  CanonicalizePath(&path);
+  EXPECT_EQ("/..", path);
+
+  path = "/../";
+  CanonicalizePath(&path);
+  EXPECT_EQ("/..", path);
+
+  path = "/../..";
+  CanonicalizePath(&path);
+  EXPECT_EQ("/../..", path);
+
+  path = "/../../";
+  CanonicalizePath(&path);
+  EXPECT_EQ("/../..", path);
+
   path = "/";
   CanonicalizePath(&path);
-  EXPECT_EQ("", path);
+  EXPECT_EQ("/", path);
 
   path = "/foo/..";
   CanonicalizePath(&path);
-  EXPECT_EQ("", path);
+  EXPECT_EQ("/", path);
 
   path = ".";
   CanonicalizePath(&path);
@@ -171,7 +215,7 @@ TEST(CanonicalizePath, PathSamplesWindows) {
 
   path = "\\";
   CanonicalizePath(&path);
-  EXPECT_EQ("", path);
+  EXPECT_EQ("/", path);
 }
 
 TEST(CanonicalizePath, SlashTracking) {
@@ -321,8 +365,53 @@ TEST(CanonicalizePath, TooManyComponents) {
   EXPECT_EQ(58, std::count(path.begin(), path.end(), '\\'));
   CanonicalizePath(&path, &slash_bits);
   EXPECT_EQ(slash_bits, 0x3ffffffffffffff);
+
+  // More than 60 components is now completely ok too.
+  path =
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\"
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\"
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\"
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\"
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\"
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\"
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\"
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\"
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\"
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\"
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\"
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\"
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\"
+      "a\\a\\a\\a\\a\\a\\a\\a\\a\\x\\y.h";
+  EXPECT_EQ(218, std::count(path.begin(), path.end(), '\\'));
+  CanonicalizePath(&path, &slash_bits);
+  EXPECT_EQ(slash_bits, 0xffffffffffffffff);
 }
-#endif
+#else   // !_WIN32
+TEST(CanonicalizePath, TooManyComponents) {
+  string path;
+  uint64_t slash_bits;
+
+  // More than 60 components is now completely ok.
+  path =
+      "a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/"
+      "a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/"
+      "a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/"
+      "a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/"
+      "a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/"
+      "a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/"
+      "a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/"
+      "a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/"
+      "a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/"
+      "a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/"
+      "a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/"
+      "a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/"
+      "a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/a/"
+      "a/a/a/a/a/a/a/a/a/x/y.h";
+  EXPECT_EQ(218, std::count(path.begin(), path.end(), '/'));
+  CanonicalizePath(&path, &slash_bits);
+  EXPECT_EQ(slash_bits, 0x0);
+}
+#endif  // !_WIN32
 
 TEST(CanonicalizePath, UpDir) {
   string path, err;
@@ -353,11 +442,13 @@ TEST(CanonicalizePath, NotNullTerminated) {
   EXPECT_EQ(strlen("foo"), len);
   EXPECT_EQ("foo/. bar/.", string(path));
 
+  // Verify that foo/..file gets canonicalized to 'file' without
+  // touching the rest of the string.
   path = "foo/../file bar/.";
   len = strlen("foo/../file");
   CanonicalizePath(&path[0], &len, &unused);
   EXPECT_EQ(strlen("file"), len);
-  EXPECT_EQ("file ./file bar/.", string(path));
+  EXPECT_EQ("file../file bar/.", string(path));
 }
 
 TEST(PathEscaping, TortureTest) {