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diff --git a/utils/abtest.py b/utils/abtest.py
new file mode 100755
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+++ b/utils/abtest.py
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+#!/usr/bin/env python
+#
+# Given a previous good compile narrow down miscompiles.
+# Expects two directories named "before" and "after" each containing a set of
+# assembly or object files where the "after" version is assumed to be broken.
+# You also have to provide a script called "link_test". It is called with a list
+# of files which should be linked together and result tested. "link_test" should
+# returns with exitcode 0 if the linking and testing succeeded.
+#
+# abtest.py operates by taking all files from the "before" directory and
+# in each step replacing one of them with a file from the "bad" directory.
+#
+# Additionally you can perform the same steps with a single .s file. In this
+# mode functions are identified by " -- Begin function FunctionName" and
+# " -- End function" markers. The abtest.py then takes all
+# function from the file in the "before" directory and replaces one function
+# with the corresponding function from the "bad" file in each step.
+#
+# Example usage to identify miscompiled files:
+#    1. Create a link_test script, make it executable. Simple Example:
+#          clang "$@" -o /tmp/test && /tmp/test || echo "PROBLEM"
+#    2. Run the script to figure out which files are miscompiled:
+#       > ./abtest.py 
+#       somefile.s: ok
+#       someotherfile.s: skipped: same content
+#       anotherfile.s: failed: './link_test' exitcode != 0
+#       ...
+# Example usage to identify miscompiled functions inside a file:
+#    3. Run the tests on a single file (assuming before/file.s and
+#       after/file.s exist)
+#       > ./abtest.py file.s
+#       funcname1 [0/XX]: ok
+#       funcname2 [1/XX]: ok
+#       funcname3 [2/XX]: skipped: same content
+#       funcname4 [3/XX]: failed: './link_test' exitcode != 0
+#       ...
+from fnmatch import filter
+from sys import stderr
+import argparse
+import filecmp
+import os
+import subprocess
+import sys
+
+LINKTEST="./link_test"
+ESCAPE="\033[%sm"
+BOLD=ESCAPE % "1"
+RED=ESCAPE % "31"
+NORMAL=ESCAPE % "0"
+FAILED=RED+"failed"+NORMAL
+
+def find(dir, file_filter=None):
+    files = [walkdir[0]+"/"+file for walkdir in os.walk(dir) for file in walkdir[2]]
+    if file_filter != None:
+        files = filter(files, file_filter)
+    return files
+
+def error(message):
+    stderr.write("Error: %s\n" % (message,))
+
+def warn(message):
+    stderr.write("Warning: %s\n" % (message,))
+
+def extract_functions(file):
+    functions = []
+    in_function = None
+    for line in open(file):
+        marker = line.find(" -- Begin function ")
+        if marker != -1:
+            if in_function != None:
+                warn("Missing end of function %s" % (in_function,))
+            funcname = line[marker + 19:-1]
+            in_function = funcname
+            text = line
+            continue
+
+        marker = line.find(" -- End function")
+        if marker != -1:
+            text += line
+            functions.append( (in_function, text) )
+            in_function = None
+            continue
+
+        if in_function != None:
+            text += line
+    return functions
+
+def replace_function(file, function, replacement, dest):
+    out = open(dest, "w")
+    skip = False
+    found = False
+    in_function = None
+    for line in open(file):
+        marker = line.find(" -- Begin function ")
+        if marker != -1:
+            if in_function != None:
+                warn("Missing end of function %s" % (in_function,))
+            funcname = line[marker + 19:-1]
+            in_function = funcname
+            if in_function == function:
+                out.write(replacement)
+                skip = True
+        else:
+            marker = line.find(" -- End function")
+            if marker != -1:
+                in_function = None
+                if skip:
+                    skip = False
+                    continue
+
+        if not skip:
+            out.write(line)
+
+def announce_test(name):
+    stderr.write("%s%s%s: " % (BOLD, name, NORMAL))
+    stderr.flush()
+
+def announce_result(result, info):
+    stderr.write(result)
+    if info != "":
+        stderr.write(": %s" % info)
+    stderr.write("\n")
+    stderr.flush()
+
+def testrun(files):
+    linkline="%s %s" % (LINKTEST, " ".join(files),)
+    res = subprocess.call(linkline, shell=True)
+    if res != 0:
+        announce_result(FAILED, "'%s' exitcode != 0" % LINKTEST)
+        return False
+    else:
+        announce_result("ok", "")
+        return True
+
+def check_files():
+    """Check files mode"""
+    for i in range(0, len(NO_PREFIX)):
+        f = NO_PREFIX[i]
+        b=baddir+"/"+f
+        if b not in BAD_FILES:
+            warn("There is no corresponding file to '%s' in %s" \
+                 % (gooddir+"/"+f, baddir))
+            continue
+
+        announce_test(f + " [%s/%s]" % (i+1, len(NO_PREFIX)))
+
+        # combine files (everything from good except f)
+        testfiles=[]
+        skip=False
+        for c in NO_PREFIX:
+            badfile = baddir+"/"+c
+            goodfile = gooddir+"/"+c
+            if c == f:
+                testfiles.append(badfile)
+                if filecmp.cmp(goodfile, badfile):
+                    announce_result("skipped", "same content")
+                    skip = True
+                    break
+            else:
+                testfiles.append(goodfile)
+        if skip:
+            continue
+        testrun(testfiles)
+
+def check_functions_in_file(base, goodfile, badfile):
+    functions = extract_functions(goodfile)
+    if len(functions) == 0:
+        warn("Couldn't find any function in %s, missing annotations?" % (goodfile,))
+        return
+    badfunctions = dict(extract_functions(badfile))
+    if len(functions) == 0:
+        warn("Couldn't find any function in %s, missing annotations?" % (badfile,))
+        return
+
+    COMBINED="/tmp/combined.s"
+    i = 0
+    for (func,func_text) in functions:
+        announce_test(func + " [%s/%s]" % (i+1, len(functions)))
+        i+=1
+        if func not in badfunctions:
+            warn("Function '%s' missing from bad file" % func)
+            continue
+        if badfunctions[func] == func_text:
+            announce_result("skipped", "same content")
+            continue
+        replace_function(goodfile, func, badfunctions[func], COMBINED)
+        testfiles=[]
+        for c in NO_PREFIX:
+            if c == base:
+                testfiles.append(COMBINED)
+                continue
+            testfiles.append(gooddir + "/" + c)
+
+        testrun(testfiles)
+
+parser = argparse.ArgumentParser()
+parser.add_argument('--a', dest='dir_a', default='before')
+parser.add_argument('--b', dest='dir_b', default='after')
+parser.add_argument('--insane', help='Skip sanity check', action='store_true')
+parser.add_argument('file', metavar='file', nargs='?')
+config = parser.parse_args()
+
+gooddir=config.dir_a
+baddir=config.dir_b
+
+BAD_FILES=find(baddir, "*")
+GOOD_FILES=find(gooddir, "*")
+NO_PREFIX=sorted([x[len(gooddir)+1:] for x in GOOD_FILES])
+
+# "Checking whether build environment is sane ..."
+if not config.insane:
+    announce_test("sanity check")
+    if not os.access(LINKTEST, os.X_OK):
+        error("Expect '%s' to be present and executable" % (LINKTEST,))
+        exit(1)
+
+    res = testrun(GOOD_FILES)
+    if not res:
+        # "build environment is grinning and holding a spatula. Guess not."
+        linkline="%s %s" % (LINKTEST, " ".join(GOOD_FILES),)
+        stderr.write("\n%s\n\n" % linkline)
+        stderr.write("Returned with exitcode != 0\n")
+        sys.exit(1)
+
+if config.file is not None:
+    # File exchange mode
+    goodfile = gooddir+"/"+config.file
+    badfile = baddir+"/"+config.file
+    check_functions_in_file(config.file, goodfile, badfile)
+else:
+    # Function exchange mode
+    check_files()