From 6bca0ec355aae2955c7de1cd16c2dc0dfe46f19c Mon Sep 17 00:00:00 2001
From: Sai Praveen Bangaru <31557731+saipraveenb25@users.noreply.github.com>
Date: Tue, 21 Feb 2023 11:37:15 -0500
Subject: Added support for simple while loops (#2667)

* Added support for simple while loops

* Fix support for while loops by changing logic to grab the loop update block
---
 source/slang/slang-ir-autodiff-cfg-norm.cpp | 16 ++++++++++------
 source/slang/slang-ir-autodiff-unzip.h      | 24 ++++++++++++++++++------
 2 files changed, 28 insertions(+), 12 deletions(-)

(limited to 'source')

diff --git a/source/slang/slang-ir-autodiff-cfg-norm.cpp b/source/slang/slang-ir-autodiff-cfg-norm.cpp
index 4014473ea..d64c6d1f6 100644
--- a/source/slang/slang-ir-autodiff-cfg-norm.cpp
+++ b/source/slang/slang-ir-autodiff-cfg-norm.cpp
@@ -473,14 +473,18 @@ struct CFGNormalizationPass
                     loopEndPoint = falseEndPoint;
                     isLoopOnTrueSide = false;
                 }
+
+                // Right now, we only support loops where the loop is on the true side of
+                // the condition. If we ever encounter the other case, fill in logic to 
+                // flip the condition.
+                //
+                SLANG_RELEASE_ASSERT(isLoopOnTrueSide);
                 
+                // Expect atleast one basic block (other than the condition block), in
+                // the loop.
+                //
                 SLANG_RELEASE_ASSERT(loopEndPoint.exitBlock);
-
-                // Special case.. the if-else of a loop needs it's
-                // after block to be pointing at the last block before
-                // it loops back to the if-else.
-                // 
-                // ifElse->afterBlock.set(loopEndPoint.exitBlock);
+                SLANG_RELEASE_ASSERT(!loopEndPoint.isRegionEmpty);
 
                 // Does the loop endpoint have both 'break' and 'base'
                 // control flows?
diff --git a/source/slang/slang-ir-autodiff-unzip.h b/source/slang/slang-ir-autodiff-unzip.h
index 944df2c81..2ccb8d8e2 100644
--- a/source/slang/slang-ir-autodiff-unzip.h
+++ b/source/slang/slang-ir-autodiff-unzip.h
@@ -277,14 +277,26 @@ struct DiffUnzipPass
 
     IRBlock* getUpdateBlock(IndexedRegion* region)
     {
-        // TODO: What if the 'continue' region has multiple
-        // blocks?
-        // We ideally want the _last_ block before control loops back.
+        auto initBlock = getInitializerBlock(region);
+
+        auto condBlock = region->firstBlock;
+
+        IRBlock* lastLoopBlock = nullptr;
+
+        for (auto predecessor : condBlock->getPredecessors())
+        {
+            if (predecessor != initBlock)
+                lastLoopBlock = predecessor;
+        }
+
+        // Should find atleast one predecessor that is _not_ the 
+        // init block (that contains the loop info). This 
+        // predecessor would be the last block in the loop
+        // before looping back to the condition.
         // 
-        SLANG_RELEASE_ASSERT(as<IRUnconditionalBranch>(
-            region->continueBlock->getTerminator())->getTargetBlock() == region->firstBlock);
+        SLANG_RELEASE_ASSERT(lastLoopBlock);
 
-        return region->continueBlock;
+        return lastLoopBlock;
     }
 
     IRBlock* getFirstLoopBodyBlock(IndexedRegion* region)
-- 
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