From 1dad6a86de55c38da5c356c2c6d81be8ff7884b1 Mon Sep 17 00:00:00 2001
From: Tim Peters <tim.peters@gmail.com>
Date: Mon, 18 Jun 2001 19:21:11 +0000
Subject: [PATCH] SF bug 434186: 0x80000000/2 != 0x80000000>>1 i_divmod:  New
 and simpler algorithm.  Old one returned gibberish on most boxes when the
 numerator was -sys.maxint-1.  Oddly enough, it worked in the release (not
 debug) build on Windows, because the compiler optimized away some tricky sign
 manipulations that were incorrect in this case. Makes you wonder <wink> ...
 Bugfix candidate.

---
 Lib/test/test_b1.py |  7 +++++++
 Objects/intobject.c | 40 +++++++++++++++++-----------------------
 2 files changed, 24 insertions(+), 23 deletions(-)

diff --git a/Lib/test/test_b1.py b/Lib/test/test_b1.py
index ea09d0be8ce..e2cc49b16ab 100644
--- a/Lib/test/test_b1.py
+++ b/Lib/test/test_b1.py
@@ -367,6 +367,13 @@ except ValueError:
 else:
     raise TestFailed, "int(%s)" % `s[1:]` + " should raise ValueError"
 
+# SF bug 434186:  0x80000000/2 != 0x80000000>>1.
+# Worked by accident in Windows release build, but failed in debug build.
+# Failed in all Linux builds.
+x = -1-sys.maxint
+if x >> 1 != x/2:
+    raise TestFailed("x >> 1 != x/2 when x == -1-sys.maxint")
+
 print 'isinstance'
 class C:
     pass
diff --git a/Objects/intobject.c b/Objects/intobject.c
index 9a1f0e62508..de28156c06d 100644
--- a/Objects/intobject.c
+++ b/Objects/intobject.c
@@ -434,38 +434,32 @@ int_mul(PyObject *v, PyObject *w)
 }
 
 static int
-i_divmod(register long xi, register long yi,
+i_divmod(register long x, register long y,
          long *p_xdivy, long *p_xmody)
 {
 	long xdivy, xmody;
 	
-	if (yi == 0) {
+	if (y == 0) {
 		PyErr_SetString(PyExc_ZeroDivisionError,
 				"integer division or modulo by zero");
 		return -1;
 	}
-	if (yi < 0) {
-		if (xi < 0) {
-			if (yi == -1 && -xi < 0) {
-				/* most negative / -1 */
-				err_ovf("integer division");
-				return -1;
-			}
-			xdivy = -xi / -yi;
-		}
-		else
-			xdivy = - (xi / -yi);
+	/* (-sys.maxint-1)/-1 is the only overflow case. */
+	if (y == -1 && x < 0 && x == -x) {
+		err_ovf("integer division");
+		return -1;
 	}
-	else {
-		if (xi < 0)
-			xdivy = - (-xi / yi);
-		else
-			xdivy = xi / yi;
-	}
-	xmody = xi - xdivy*yi;
-	if ((xmody < 0 && yi > 0) || (xmody > 0 && yi < 0)) {
-		xmody += yi;
-		xdivy -= 1;
+	xdivy = x / y;
+	xmody = x - xdivy * y;
+	/* If the signs of x and y differ, and the remainder is non-0,
+	 * C89 doesn't define whether xdivy is now the floor or the
+	 * ceiling of the infinitely precise quotient.  We want the floor,
+	 * and we have it iff the remainder's sign matches y's.
+	 */
+	if (xmody && ((y ^ xmody) < 0) /* i.e. and signs differ */) {
+		xmody += y;
+		--xdivy;
+		assert(xmody && ((y ^ xmody) >= 0));
 	}
 	*p_xdivy = xdivy;
 	*p_xmody = xmody;