From db4194c177cf4b8faaf7facbb4928394aa18d6a4 Mon Sep 17 00:00:00 2001
From: Andreas Kloeckner <inform@tiker.net>
Date: Mon, 26 Mar 2012 00:33:36 -0400
Subject: [PATCH] Avoid write races when determining whether a temporary should
 be local.

---
 loopy/preprocess.py | 50 ++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 43 insertions(+), 7 deletions(-)

diff --git a/loopy/preprocess.py b/loopy/preprocess.py
index 45d8c0a1e..97c3f3c44 100644
--- a/loopy/preprocess.py
+++ b/loopy/preprocess.py
@@ -51,18 +51,54 @@ def mark_local_temporaries(kernel):
     from loopy.symbolic import get_dependencies
 
     for temp_var in kernel.temporary_variables.itervalues():
+        # Only fill out for variables that do not yet know if they're
+        # local. (I.e. those generated by implicit temporary generation.)
+
+        if temp_var.is_local is not None:
+            new_temp_vars[temp_var.name] = temp_var
+            continue
+
         my_writers = writers[temp_var.name]
 
-        has_local_parallel_write = False
+        wants_to_be_local_per_insn = []
         for insn_id in my_writers:
             insn = kernel.id_to_insn[insn_id]
-            has_local_parallel_write = has_local_parallel_write or any(
-                    isinstance(kernel.iname_to_tag.get(iname), LocalIndexTagBase)
-                    for iname in get_dependencies(insn.get_assignee_indices())
-                    & kernel.all_inames())
 
-        new_temp_vars[temp_var.name] = temp_var.copy(
-                is_local=has_local_parallel_write)
+            # A write race will emerge if:
+            #
+            # - the variable is local
+            #   and
+            # - the instruction is run across more inames (locally) parallel
+            #   than are reflected in the assignee indices.
+
+            parallel_compute_inames = set(iname
+                    for iname in kernel.insn_inames(insn_id)
+                    if isinstance(kernel.iname_to_tag.get(iname), LocalIndexTagBase))
+
+            parallel_assignee_inames = set(iname
+                    for iname in
+                        get_dependencies(insn.get_assignee_indices())
+                        & kernel.all_inames()
+                    if isinstance(kernel.iname_to_tag.get(iname), LocalIndexTagBase))
+
+            assert parallel_assignee_inames <= parallel_compute_inames
+
+            wants_to_be_local_per_insn.append(
+                    parallel_assignee_inames == parallel_compute_inames)
+
+        if not wants_to_be_local_per_insn:
+            from warnings import warn
+            from loopy import LoopyAdvisory
+            warn("temporary variable '%s' never written, eliminating"
+                    % temp_var.name, LoopyAdvisory)
+
+        is_local = wants_to_be_local_per_insn[0]
+        from pytools import all
+        if not all(wtbl==is_local for wtbl in wants_to_be_local_per_insn):
+            raise RuntimeError("not all instructions agree on whether "
+                    "temporary '%s' should be in local memory" % temp_var.name)
+
+        new_temp_vars[temp_var.name] = temp_var.copy(is_local=is_local)
 
     return kernel.copy(temporary_variables=new_temp_vars)
 
-- 
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