[llvm] [SCEVExpander] Relax hoisting condition for AddRec start (PR #75916)

[via llvm-commits]

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@@ -980,11 +980,14 @@ SCEVExpander::getAddRecExprPHILiterally(const SCEVAddRecExpr *Normalized,
   PostIncLoopSet SavedPostIncLoops = PostIncLoops;
   PostIncLoops.clear();
 
-  // Expand code for the start value into the loop preheader.
-  assert(L->getLoopPreheader() &&
-         "Can't expand add recurrences without a loop preheader!");
-  Value *StartV =
-      expand(Normalized->getStart(), L->getLoopPreheader()->getTerminator());
+  // Expand code for the start value into the loop predecessor. The loop is not
+  // necessarily in Loop Simplify Form, so assert it's legal to do so.
+  auto *LP = L->getLoopPredecessor();
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averine wrote:

No: At that line 985, I cannot guarantee that `LP` dominates the loop body.
This is "checked" this in the `assert` line 986 (by definition of `getLoopPredecessor()`, `LP` is the unique predecessor of the header iff it's non-zero).

So the patch helps when `L` has no preheader but a unique predecessor (which is legal to hoist into), but leads to the same error when `L` has several predecessors.

https://github.com/llvm/llvm-project/pull/75916