CGP: Clear data structures at the end of a loop iteration instead of the beginning.

[pcc]

Summary

Clearing LargeOffsetGEPMap at the end fixes a bug where if a large
offset GEP is in a dead basic block, we fail an assertion when trying
to delete the block due to the asserting VH in LargeOffsetGEPMap.

[efriedma]

LGTM

llvm/lib/CodeGen/CodeGenPrepare.cpp

bool CodeGenPrepare::runOnFunction(Function &F) {
 
   // Split some critical edges where one of the sources is an indirect branch,
   // to help generate sane code for PHIs involving such edges.
   EverMadeChange |= SplitIndirectBrCriticalEdges(F);
 
   bool MadeChange = true;
   while (MadeChange) {
     MadeChange = false;
-    SeenChainsForSExt.clear();
-    ValToSExtendedUses.clear();
-    RemovedInsts.clear();
-    LargeOffsetGEPMap.clear();
-    LargeOffsetGEPID.clear();
     for (Function::iterator I = F.begin(); I != F.end(); ) {
       BasicBlock *BB = &*I++;
       bool ModifiedDTOnIteration = false;
       MadeChange |= optimizeBlock(*BB, ModifiedDTOnIteration);
 
       // Restart BB iteration if the dominator tree of the Function was changed
       if (ModifiedDTOnIteration)
         break;
     }
     if (EnableTypePromotionMerge && !ValToSExtendedUses.empty())
       MadeChange |= mergeSExts(F);
     if (!LargeOffsetGEPMap.empty())
       MadeChange |= splitLargeGEPOffsets();
 
     // Really free removed instructions during promotion.
     for (Instruction *I : RemovedInsts)
       I->deleteValue();
 
     EverMadeChange |= MadeChange;
+    SeenChainsForSExt.clear();
+    ValToSExtendedUses.clear();
+    RemovedInsts.clear();
+    LargeOffsetGEPMap.clear();
+    LargeOffsetGEPID.clear();
   }
 
   SunkAddrs.clear();
 
   if (!DisableBranchOpts) {
     MadeChange = false;
     // Use a set vector to get deterministic iteration order. The order the
     // blocks are removed may affect whether or not PHI nodes in successors

llvm/test/CodeGen/Thumb2/unreachable-large-offset-gep.ll

+; RUN: llc -o - %s | FileCheck %s
+
+; CHECK: .LBB0_1:
+; CHECK: b .LBB0_1
+
+target triple = "thumbv8m-unknown-linux-android"
+
+define void @d(i32* %c) {
+entry:
+  br i1 false, label %f.exit, label %i.d
+
+i.d:
+  br label %i.d
+
+f.exit:
+  %0 = getelementptr i32, i32* %c, i32 57
+  br label %if.g
+
+if.g:
+  store i32 0, i32* %0
+  ret void
+}