diff --git a/Makefile.in b/Makefile.in
index 03d569d..5aaedcd 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -19,11 +19,11 @@ libs = @LIBS@ -lm
 sources = \
     ccache.c mdfour.c hash.c execute.c util.c args.c stats.c \
     cleanup.c snprintf.c unify.c manifest.c hashtable.c hashtable_itr.c \
-    murmurhashneutral2.c hashutil.c
+    murmurhashneutral2.c hashutil.c comments.c
 
 headers = \
     ccache.h hashtable.h hashtable_itr.h hashtable_private.h hashutil.h \
-    manifest.h mdfour.h murmurhashneutral2.h
+    manifest.h mdfour.h murmurhashneutral2.h comments.h
 
 objs = $(sources:.c=.o)
 
diff --git a/ccache.c b/ccache.c
index 897bf08..29d74c7 100644
--- a/ccache.c
+++ b/ccache.c
@@ -27,6 +27,7 @@
 #include "hashtable_itr.h"
 #include "hashutil.h"
 #include "manifest.h"
+#include "comments.h"
 
 #include <getopt.h>
 
@@ -244,7 +245,7 @@ static void remember_include_file(char *path, size_t path_len)
 	struct mdfour fhash;
 	struct stat st;
 	int fd = -1;
-	int ret;
+	char *data = (char *)-1;
 
 	if (!included_files) {
 		goto ignore;
@@ -284,19 +285,20 @@ static void remember_include_file(char *path, size_t path_len)
 		cc_log("Include file \"%s\" too new\n", path);
 		goto failure;
 	}
-	hash_start(&fhash);
-	ret = hash_fd(&fhash, fd);
-	if (!ret) {
-		cc_log("Failed hashing include file \"%s\"\n", path);
+	data = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	if (data == (char *)-1) {
+		cc_log("Failed to mmap %s\n", path);
 		goto failure;
 	}
 
-	/* Hashing OK. */
+	hash_start(&fhash);
+	hash_string_ignoring_comments(&fhash, data, st.st_size);
+
 	h = x_malloc(sizeof(*h));
 	hash_result_as_bytes(&fhash, h->hash);
 	h->size = fhash.totalN;
 	hashtable_insert(included_files, path, h);
-	close(fd);
+	munmap(data, st.st_size);
 	return;
 
 failure:
@@ -307,6 +309,9 @@ failure:
 	/* Fall through. */
 ignore:
 	free(path);
+	if (data != (char *)-1) {
+		munmap(data, st.st_size);
+	}
 	if (fd != -1) {
 		close(fd);
 	}
@@ -786,7 +791,7 @@ static int find_hash(ARGS *args, enum findhash_call_mode mode)
 
 	switch (mode) {
 	case FINDHASH_DIRECT_MODE:
-		if (!hash_file(&hash, input_file)) {
+		if (!hash_file_ignoring_comments(&hash, input_file)) {
 			cc_log("Failed hashing %s\n", input_file);
 			failed();
 		}
@@ -980,7 +985,8 @@ static void from_cache(enum fromcache_call_mode mode, int put_object_in_manifest
 	/* Create or update the manifest file. */
 	if (put_object_in_manifest && included_files) {
 		if (manifest_put(manifest_path, object_hash, included_files)) {
-			cc_log("Added object file hash to manifest\n");
+			cc_log("Added object file hash to manifest %s\n",
+				manifest_path);
 			/* Update timestamp for LRU cleanup. */
 #ifdef HAVE_UTIMES
 			utimes(manifest_path, NULL);
diff --git a/comments.c b/comments.c
new file mode 100644
index 0000000..daf9d8f
--- /dev/null
+++ b/comments.c
@@ -0,0 +1,135 @@
+/*
+ * Copyright (C) Joel Rosdahl 2009
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program; if not, write to the Free Software Foundation, Inc., 675 Mass
+ * Ave, Cambridge, MA 02139, USA.
+ */
+
+#include "ccache.h"
+#include "comments.h"
+
+#define HASH(ch)							\
+	do {								\
+		hashbuf[hashbuflen] = ch;				\
+		hashbuflen++;						\
+		if (hashbuflen == sizeof(hashbuf)) {			\
+			hash_buffer(hash, hashbuf, sizeof(hashbuf));	\
+			hashbuflen = 0;					\
+		}							\
+	} while (0)
+
+void hash_string_ignoring_comments(
+	struct mdfour *hash, const char *str, size_t len)
+{
+	const char *p;
+	const char *end;
+	char hashbuf[64];
+	size_t hashbuflen = 0;
+
+	p = str;
+	end = str + len;
+	while (1) {
+		if (p >= end) {
+			goto end;
+		}
+		switch (*p) {
+		case '/':
+			if (p+1 == end) {
+				break;
+			}
+			switch (*(p+1)) {
+			case '*':
+				HASH(' '); /* Don't paste tokens together when
+					    * removing the comment. */
+				p += 2;
+				while (p+1 < end
+				       && (*p != '*' || *(p+1) != '/')) {
+					if (*p == '\n') {
+						/* Keep line numbers. */
+						HASH('\n');
+					}
+					p++;
+				}
+				if (p+1 == end) {
+					goto end;
+				}
+				p += 2;
+				continue;
+
+			case '/':
+				p += 2;
+				while (p < end
+				       && (*p != '\n' || *(p-1) == '\\')) {
+					p++;
+				}
+				continue;
+
+			default:
+				break;
+			}
+			break;
+
+		case '"':
+			HASH(*p);
+			p++;
+			while (p < end && (*p != '"' || *(p-1) == '\\')) {
+				HASH(*p);
+				p++;
+			}
+			if (p == end) {
+				goto end;
+			}
+			break;
+
+		default:
+			break;
+		}
+
+		HASH(*p);
+		p++;
+	}
+
+end:
+	hash_buffer(hash, hashbuf, hashbuflen);
+}
+
+/*
+ * Add contents of a file to a hash, but don't hash comments. Returns 1 on
+ * success, otherwise 0.
+ */
+int hash_file_ignoring_comments(struct mdfour *hash, const char *path)
+{
+	int fd;
+	struct stat st;
+	char *data;
+
+	fd = open(path, O_RDONLY);
+	if (fd == -1) {
+		return 0;
+	}
+	if (fstat(fd, &st) == -1) {
+		close(fd);
+		return 0;
+	}
+	data = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	close(fd);
+	if (data == (void *)-1) {
+		return 0;
+	}
+
+	hash_string_ignoring_comments(hash, data, st.st_size);
+
+	munmap(data, st.st_size);
+	return 1;
+}
diff --git a/comments.h b/comments.h
new file mode 100644
index 0000000..bbc3f06
--- /dev/null
+++ b/comments.h
@@ -0,0 +1,10 @@
+#ifndef COMMENTS_H
+#define COMMENTS_H
+
+#include "mdfour.h"
+
+void hash_string_ignoring_comments(
+	struct mdfour *hash, const char *str, size_t len);
+int hash_file_ignoring_comments(struct mdfour *hash, const char *path);
+
+#endif
diff --git a/manifest.c b/manifest.c
index 9a17113..31f5b6b 100644
--- a/manifest.c
+++ b/manifest.c
@@ -29,6 +29,7 @@
 #include "hashutil.h"
 #include "manifest.h"
 #include "murmurhashneutral2.h"
+#include "comments.h"
 
 extern char *temp_dir;
 
@@ -345,7 +346,8 @@ static int verify_object(struct manifest *mf, struct object *obj,
 		if (!actual) {
 			actual = x_malloc(sizeof(*actual));
 			hash_start(&hash);
-			if (!hash_file(&hash, mf->files[fi->index])) {
+			if (!hash_file_ignoring_comments(
+				    &hash, mf->files[fi->index])) {
 				cc_log("Failed hashing %s\n",
 				       mf->files[fi->index]);
 				free(actual);
diff --git a/test.sh b/test.sh
index e8cf31e..d35b3ad 100755
--- a/test.sh
+++ b/test.sh
@@ -442,6 +442,7 @@ EOF
     ##################################################################
     # Check that -Wp,-MMD,file.d works.
     testname="-Wp,-MMD"
+    $CCACHE -C >/dev/null
     $CCACHE -z >/dev/null
     $CCACHE $COMPILER -c -Wp,-MMD,other.d test.c
     checkstat 'cache hit (direct)' 0
@@ -530,6 +531,7 @@ EOF
     ##################################################################
     # Check that -MF works.
     testname="-MF"
+    $CCACHE -C >/dev/null
     $CCACHE -z >/dev/null
     $CCACHE $COMPILER -c -MD -MF other.d test.c
     checkstat 'cache hit (direct)' 0
@@ -603,6 +605,45 @@ EOF
     checkstat 'cache miss' 1
     checkfile stderr-mf.txt "`cat stderr-orig.txt`"
 
+    ##################################################################
+    # Check that changes in comments are ignored when hashing.
+    testname="changes in comments"
+    $CCACHE -C >/dev/null
+    $CCACHE -z >/dev/null
+    cat <<EOF >comments.h
+/*
+ * /* foo comment
+ */
+EOF
+    cat <<'EOF' >comments.c
+#include "comments.h"
+char test[] = "\
+/* apple */ // banana"; // foo comment
+EOF
+    sleep 1 # Sleep to make the include file trusted.
+
+    $CCACHE $COMPILER -c comments.c
+    checkstat 'cache hit (direct)' 0
+    checkstat 'cache hit (preprocessed)' 0
+    checkstat 'cache miss' 1
+
+    sed -i 's/foo/ignored/' comments.h comments.c
+    sleep 1 # Sleep to make the include file trusted.
+
+    $CCACHE $COMPILER -c comments.c
+    checkstat 'cache hit (direct)' 1
+    checkstat 'cache hit (preprocessed)' 0
+    checkstat 'cache miss' 1
+
+    # Check that comment-like string contents are hashed.
+    sed -i 's/apple/orange/' comments.c
+    sleep 1 # Sleep to make the include file trusted.
+
+    $CCACHE $COMPILER -c comments.c
+    checkstat 'cache hit (direct)' 1
+    checkstat 'cache hit (preprocessed)' 0
+    checkstat 'cache miss' 2
+
     ##################################################################
     # Reset things.
     CCACHE_NODIRECT=1