Subzero. Fixes a bug in the register allocator.

This bug was uncovered While implementing the llvm.nacl.atomic.cmpxchg lowering for i64 for ARM32. For reference, the lowering is retry: ldrexd tmp_i, tmp_i+1 [addr] cmp tmp_i+1, expected_i+1 cmpeq tmp_i, expected_i strexdeq success, new_i, new_i+1, [addr] movne expected_i+1, tmp_i+1 movne expected_i, tmp_i cmpeq success, #0 bne retry mov dest_i+1, tmp_i+1 mov dest_i, tmp_i The register allocator would allocate r4 to both success and new_i, which is clearly wrong (expected_i is alive thought the cmpxchg loop.) Adding a fake-use(new_i) after the loop caused the register allocator to fail due to the impossibility to allocate a register for an infinite weight register. The problem was being caused for not evicting live ranges that were assigned registers that alias the selected register. BUG= R=kschimpf@google.com, stichnot@chromium.org Review URL: https://codereview.chromium.org/1373823006 .

Subzero. Fixes a bug in the register allocator.
7cb12682 · John Porto · 16991847 · 7cb12682
Commit 7cb12682 authored Oct 01, 2015 by John Porto
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Showing with 8 additions and 12 deletions

IceRegAlloc.cpp src/IceRegAlloc.cpp +8 -12

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--- a/src/IceRegAlloc.cpp
+++ b/src/IceRegAlloc.cpp
@@ -652,17 +652,15 @@ void LinearScan::handleNoFreeRegisters(IterationState &Iter) {
  } else {
    // Evict all live ranges in Active that register number MinWeightIndex is
    // assigned to.
+    const llvm::SmallBitVector &Aliases = *RegAliases[MinWeightIndex];
    for (SizeT I = Active.size(); I > 0; --I) {
      const SizeT Index = I - 1;
      Variable *Item = Active[Index];
-      if (Item->getRegNumTmp() == MinWeightIndex) {
+      int32_t RegNum = Item->getRegNumTmp();
-        dumpLiveRangeTrace("Evicting     ", Item);
+      if (Aliases[RegNum]) {
-        const llvm::SmallBitVector &Aliases = *RegAliases[MinWeightIndex];
+        dumpLiveRangeTrace("Evicting A   ", Item);
-        for (int32_t RegAlias = Aliases.find_first(); RegAlias >= 0;
+        --RegUses[RegNum];
-             RegAlias = Aliases.find_next(RegAlias)) {
+        assert(RegUses[RegNum] >= 0);
-          --RegUses[RegAlias];
-          assert(RegUses[RegAlias] >= 0);
-        }
        Item->setRegNumTmp(Variable::NoRegister);
        moveItem(Active, Index, Handled);
      }
@@ -678,16 +676,14 @@ void LinearScan::handleNoFreeRegisters(IterationState &Iter) {
      // especially bad if we would end up evicting an infinite-weight but
      // currently-inactive live range. The most common situation for this
      // would be a scratch register kill set for call instructions.
-      if (Item->getRegNumTmp() == MinWeightIndex &&
+      if (Aliases[Item->getRegNumTmp()] && Item->rangeOverlaps(Iter.Cur)) {
-          Item->rangeOverlaps(Iter.Cur)) {
+        dumpLiveRangeTrace("Evicting I   ", Item);
-        dumpLiveRangeTrace("Evicting     ", Item);
        Item->setRegNumTmp(Variable::NoRegister);
        moveItem(Inactive, Index, Handled);
      }
    }
    // Assign the register to Cur.
    Iter.Cur->setRegNumTmp(MinWeightIndex);
-    const llvm::SmallBitVector &Aliases = *RegAliases[MinWeightIndex];
    for (int32_t RegAlias = Aliases.find_first(); RegAlias >= 0;
         RegAlias = Aliases.find_next(RegAlias)) {
      assert(RegUses[RegAlias] >= 0);