From 17caa6ede3b36d0a867652a14a2abca004b0c0c6 Mon Sep 17 00:00:00 2001
From: Andreas Kloeckner <inform@tiker.net>
Date: Mon, 30 Dec 2013 15:38:10 +0100
Subject: [PATCH] Add common problem to tutorial: 'a static maximum was not
found'
---
doc/tutorial.rst | 51 ++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 51 insertions(+)
diff --git a/doc/tutorial.rst b/doc/tutorial.rst
index 063e497fb..f4cd4af24 100644
--- a/doc/tutorial.rst
+++ b/doc/tutorial.rst
@@ -429,4 +429,55 @@ Specifying arguments
Implementing Array Axes
-----------------------
+Common Problems
+---------------
+
+A static maximum was not found
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Attempting to create this kernel results in an error:
+
+.. doctest::
+ :options: +ELLIPSIS
+
+ >>> knl = lp.make_kernel(ctx.devices[0],
+ ... "{ [i]: 0<=i<n }",
+ ... """
+ ... out[i] = 5
+ ... out[0] = 6
+ ... """)
+ ... # Loopy prints the following before this exception:
+ ... # While trying to find shape axis 0 of argument 'out', the following exception occurred:
+ Traceback (most recent call last):
+ ...
+ ValueError: a static maximum was not found for PwAff '[n] -> { [(1)] : n = 1; [(n)] : n >= 2; [(1)] : n <= 0 }'
+
+The problem is that loopy cannot find a simple, universally valid expression
+for the length of *out* in this case. The set notation at the end of the error
+message summarizes its best attempt:
+
+* If n=1, then out has size 1.
+* If n>=2, then out has size n.
+* If n<=0, then out has size 1.
+
+Sure, some of these cases could be coalesced, but that's beside the point.
+Loopy does not know that non-positive values of *n* make no sense. It needs to
+be told in order for the error to disappear--note the *assumptions* argument:
+
+.. doctest::
+ :options: +ELLIPSIS
+
+ >>> knl = lp.make_kernel(ctx.devices[0],
+ ... "{ [i]: 0<=i<n }",
+ ... """
+ ... out[i] = 5
+ ... out[0] = 6
+ ... """, assumptions="n>=1")
+
+Other situations where this error message can occur include:
+
+* Finding size of prefetch/precompute arrays
+* Finding sizes of argument arrays
+* Finding workgroup sizes
+
.. vim: tw=75:spell
--
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