Recommit the fix for rdar://9289512 with a couple tweaks to
fix bugs exposed by the gcc dejagnu testsuite:
1. The load may actually be used by a dead instruction, which
would cause an assert.
2. The load may not be used by the current chain of instructions,
and we could move it past a side-effecting instruction. Change
how we process uses to define the problem away.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130018 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index 25dd97d..fdf3767 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -746,16 +746,49 @@
-
+/// TryToFoldFastISelLoad - We're checking to see if we can fold the specified
+/// load into the specified FoldInst. Note that we could have a sequence where
+/// multiple LLVM IR instructions are folded into the same machineinstr. For
+/// example we could have:
+/// A: x = load i32 *P
+/// B: y = icmp A, 42
+/// C: br y, ...
+///
+/// In this scenario, LI is "A", and FoldInst is "C". We know about "B" (and
+/// any other folded instructions) because it is between A and C.
+///
+/// If we succeed in folding the load into the operation, return true.
+///
bool SelectionDAGISel::TryToFoldFastISelLoad(const LoadInst *LI,
+ const Instruction *FoldInst,
FastISel *FastIS) {
+ // We know that the load has a single use, but don't know what it is. If it
+ // isn't one of the folded instructions, then we can't succeed here. Handle
+ // this by scanning the single-use users of the load until we get to FoldInst.
+ unsigned MaxUsers = 6; // Don't scan down huge single-use chains of instrs.
+
+ const Instruction *TheUser = LI->use_back();
+ while (TheUser != FoldInst && // Scan up until we find FoldInst.
+ // Stay in the right block.
+ TheUser->getParent() == FoldInst->getParent() &&
+ --MaxUsers) { // Don't scan too far.
+ // If there are multiple or no uses of this instruction, then bail out.
+ if (!TheUser->hasOneUse())
+ return false;
+
+ TheUser = TheUser->use_back();
+ }
+
// Don't try to fold volatile loads. Target has to deal with alignment
// constraints.
if (LI->isVolatile()) return false;
- // Figure out which vreg this is going into.
+ // Figure out which vreg this is going into. If there is no assigned vreg yet
+ // then there actually was no reference to it. Perhaps the load is referenced
+ // by a dead instruction.
unsigned LoadReg = FastIS->getRegForValue(LI);
- assert(LoadReg && "Load isn't already assigned a vreg? ");
+ if (LoadReg == 0)
+ return false;
// Check to see what the uses of this vreg are. If it has no uses, or more
// than one use (at the machine instr level) then we can't fold it.
@@ -833,10 +866,10 @@
/// Return false if it needs to be emitted.
static bool isFoldedOrDeadInstruction(const Instruction *I,
FunctionLoweringInfo *FuncInfo) {
- return !I->mayWriteToMemory() &&
- !isa<TerminatorInst>(I) &&
- !isa<DbgInfoIntrinsic>(I) &&
- !FuncInfo->isExportedInst(I);
+ return !I->mayWriteToMemory() && // Side-effecting instructions aren't folded.
+ !isa<TerminatorInst>(I) && // Terminators aren't folded.
+ !isa<DbgInfoIntrinsic>(I) && // Debug instructions aren't folded.
+ !FuncInfo->isExportedInst(I); // Exported instrs must be computed.
}
void SelectionDAGISel::SelectAllBasicBlocks(const Function &Fn) {
@@ -928,16 +961,20 @@
// Try to select the instruction with FastISel.
if (FastIS->SelectInstruction(Inst)) {
- // If fast isel succeeded, check to see if there is a single-use
- // non-volatile load right before the selected instruction, and see if
- // the load is used by the instruction. If so, try to fold it.
- const Instruction *BeforeInst = 0;
- if (Inst != Begin)
- BeforeInst = llvm::prior(llvm::prior(BI));
- if (BeforeInst && isa<LoadInst>(BeforeInst) &&
- BeforeInst->hasOneUse() && *BeforeInst->use_begin() == Inst &&
- TryToFoldFastISelLoad(cast<LoadInst>(BeforeInst), FastIS))
- --BI; // If we succeeded, don't re-select the load.
+ // If fast isel succeeded, skip over all the folded instructions, and
+ // then see if there is a load right before the selected instructions.
+ // Try to fold the load if so.
+ const Instruction *BeforeInst = Inst;
+ while (BeforeInst != Begin) {
+ BeforeInst = llvm::prior(BasicBlock::const_iterator(BeforeInst));
+ if (!isFoldedOrDeadInstruction(BeforeInst, FuncInfo))
+ break;
+ }
+ if (BeforeInst != Inst && isa<LoadInst>(BeforeInst) &&
+ BeforeInst->hasOneUse() &&
+ TryToFoldFastISelLoad(cast<LoadInst>(BeforeInst), Inst, FastIS))
+ // If we succeeded, don't re-select the load.
+ BI = llvm::next(BasicBlock::const_iterator(BeforeInst));
continue;
}