[MSSA] Rename uses in IDF regardless of new def position in basic block.

[asbirlea]

Summary

When inserting a new def and renaming of uses is asked, always compute
IDF and do the renaming for the blocks with Phis in that IDF.
Resolves PR49859.

[fhahn]

Thanks for putting up the patch! Looks compile-time neutral: http://llvm-compile-time-tracker.com/compare.php?from=0494b6b6769f9ceda7e019a30e83a472d23afff5&to=1e6ad8da46f1cf45300df0f2083c1636e34e54c8&stat=instructions

[george.burgess.iv]

Thanks for this!

[uabelho]

Great, thanks for the fix!

Just a question, any reason not to include a regression test, e.g. the reproducer in PR49859?

[asbirlea]

Test added in 1e0b813fc082.

[uabelho]

In D100163#2686398, @asbirlea wrote:

Test added in 1e0b813fc082.

Nice, thanks!

llvm/lib/Analysis/MemorySSAUpdater.cpp

if (!DefBeforeSameBlock) {
     // backwards to find the def.  To make that work, we'd have to track whether
     // getDefRecursive only ever used the single predecessor case.  These types
     // of paths also only exist in between CFG simplifications.
 
     // If this is the first def in the block and this insert is in an arbitrary
     // place, compute IDF and place phis.
     SmallPtrSet<BasicBlock *, 2> DefiningBlocks;
 
-    // If this is the last Def in the block, also compute IDF based on MD, since
-    // this may a new Def added, and we may need additional Phis.
-    auto Iter = MD->getDefsIterator();
-    ++Iter;
-    auto IterEnd = MSSA->getBlockDefs(MD->getBlock())->end();
-    if (Iter == IterEnd)
-      DefiningBlocks.insert(MD->getBlock());
-
+    // If this is the last Def in the block, we may need additional Phis.
+    // Compute IDF in all cases, as renaming needs to be done even when MD is
+    // not the last access, because it can introduce a new access past which a
+    // previous access was optimized; that access needs to be reoptimized.
     for (const auto &VH : InsertedPHIs)
       if (const auto *RealPHI = cast_or_null<MemoryPhi>(VH))
         DefiningBlocks.insert(RealPHI->getBlock());
     ForwardIDFCalculator IDFs(*MSSA->DT);
     SmallVector<BasicBlock *, 32> IDFBlocks;
     IDFs.setDefiningBlocks(DefiningBlocks);
     IDFs.calculate(IDFBlocks);
     SmallVector<AssertingVH<MemoryPhi>, 4> NewInsertedPHIs;