CodeGen: BlockPlacement: Clear ComputedEdges between functions.

[iteratee]

Summary

Not clearing was causing non-deterministic compiles for large files. Addresses
for MachineBasicBlocks would end up colliding and we would lay out a block that
we assumed had been pre-computed when it had not been.

[chandlerc]

LGTM. Also, this is clearly correct, feel free to land this simple of bug fix directly. =]

It'd be great if you can come up with a test with a pile of functions that will trigger this, but I understand why that might be hard. The assert seems likely enough.

[Gerolf]

Please double check your code one more time. Thank you!

-Gerolf

lib/CodeGen/MachineBlockPlacement.cpp

2675

I think you need to clear ComptedEdges here for the same reason you clear BlockToChain.

[iteratee]

I found the bugs mentioned via my own testing.

[iteratee]

Already committed in rL300022

[iteratee]

Gerolf if you could accept this so I can close it, I would appreciate it.

[iteratee]

Committed in rL300022. The committed version contained a fix for the error Gerolf spotted.

[Gerolf]

Thanks, Kyle!

[echristo]

llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp

2643 ↗

(On Diff #97524)

Drive-by nit: Typically we like to have some text in the assert as well to explain why we're checking for such a thing, e.g. assert(BlockToChain.empty() && "Forgot to clear BlockToChain between functions!") or something.

lib/CodeGen/MachineBlockPlacement.cpp

if (TailDupPlacement) {
     unsigned TailDupSize = TailDupPlacementThreshold;
     if (MF.getFunction()->optForSize())
       TailDupSize = 1;
     TailDup.initMF(MF, MBPI, /* LayoutMode */ true, TailDupSize);
     precomputeTriangleChains();
   }
 
   assert(BlockToChain.empty());
+  assert(ComputedEdges.empty());
 
   buildCFGChains();
 
   // Changing the layout can create new tail merging opportunities.
   TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
   // TailMerge can create jump into if branches that make CFG irreducible for
   // HW that requires structured CFG.
   bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
                          PassConfig->getEnableTailMerge() &&
                          BranchFoldPlacement;
   // No tail merging opportunities if the block number is less than four.
   if (MF.size() > 3 && EnableTailMerge) {
     unsigned TailMergeSize = TailDupPlacementThreshold + 1;
     BranchFolder BF(/*EnableTailMerge=*/true, /*CommonHoist=*/false, *MBFI,
                     *MBPI, TailMergeSize);
 
     if (BF.OptimizeFunction(MF, TII, MF.getSubtarget().getRegisterInfo(),
                             getAnalysisIfAvailable<MachineModuleInfo>(), MLI,
                             /*AfterBlockPlacement=*/true)) {
       // Redo the layout if tail merging creates/removes/moves blocks.
       BlockToChain.clear();
       // Must redo the post-dominator tree if blocks were changed.
       if (MPDT)
         MPDT->runOnMachineFunction(MF);
       ChainAllocator.DestroyAll();
       buildCFGChains();
     }
   }
 
   optimizeBranches();
   alignBlocks();
 
   BlockToChain.clear();
+  ComputedEdges.clear();
   ChainAllocator.DestroyAll();
 
   if (AlignAllBlock)
     // Align all of the blocks in the function to a specific alignment.
     for (MachineBasicBlock &MBB : MF)
       MBB.setAlignment(AlignAllBlock);
   else if (AlignAllNonFallThruBlocks) {
     // Align all of the blocks that have no fall-through predecessors to a