Fix CFG reversal logic for loops (#4162)

Handles a corner case where the first block after the condition on the true-side is another condition. This would currently result in an invalid reverse graph, where the reverse version of the true-block is the merge point for two different branching insts (the reverse version of the loop as well as the second condition).
This patch simply adds a blank block when constructing the reverse-loop (similar to critical edge breaking) so that each branch inst in the reversed loop has a unique merge block.

1 files changed, 27 insertions, 1 deletions

diff --git a/source/slang/slang-ir-autodiff-transpose.h b/source/slang/slang-ir-autodiff-transpose.h
index 2283ebf5c..05884d13d 100644
--- a/source/slang/slang-ir-autodiff-transpose.h
+++ b/source/slang/slang-ir-autodiff-transpose.h
@@ -320,8 +320,34 @@ struct DiffTransposePass
                     // Old cond block becomes new pre-break block.
                     IRBlock* revBreakBlock = revBlockMap[currentBlock];
 
-                    // Old true-side starting block becomes loop end block.
+                    // Old true-side starting block becomes loop end block...
                     IRBlock* revLoopEndBlock = revBlockMap[trueBlock];
+
+                    // ... unless the true block has multiple successors, in which 
+                    // case revLoopEndBlock is the merge block for some other if-else region
+                    // 
+                    // We will insert a new block after revLookEndBlock, which will serve as the
+                    // actual end block.
+                    // 
+                    HashSet<IRBlock*> uniqueSuccessors;
+                    for (auto successor : trueBlock->getSuccessors())
+                        uniqueSuccessors.add(successor);
+                    if (uniqueSuccessors.getCount() > 1)
+                    {
+                        auto revLookPreEndBlock = revLoopEndBlock;
+                        builder.setInsertAfter(revLookPreEndBlock);
+                        revLoopEndBlock = builder.emitBlock();
+
+                        if (isDifferentialInst(trueBlock))
+                        {
+                            builder.markInstAsDifferential(revLoopEndBlock);
+                        }
+
+                        builder.setInsertInto(revLookPreEndBlock);
+                        builder.emitBranch(revLoopEndBlock);
+                    }
+                    
+                    // Then, branch from the new loop end block to the new cond block.
                     builder.setInsertInto(revLoopEndBlock);
                     builder.emitBranch(
                         revCondBlock,