[ARM][LowOverheadLoops] Revert after read/write
Currently we check whether LR is stored/loaded to/from inbetween the
loop decrement and loop end pseudo instructions. There's two problems
here:
- It relies on all load/store instructions being labelled as such in
tablegen.
- Actually any use of loop decrement is troublesome because the value
doesn't exist!
So we need to check for any read/write of LR that occurs between the
two instructions and revert if we find anything.
Differential Revision: https://reviews.llvm.org/D65792
llvm-svn: 368130
diff --git a/llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp b/llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp
index 303986a..c548f36 100644
--- a/llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp
+++ b/llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp
@@ -11,8 +11,7 @@
/// The expectation is that the loop contains three pseudo instructions:
/// - t2*LoopStart - placed in the preheader or pre-preheader. The do-loop
/// form should be in the preheader, whereas the while form should be in the
-/// preheaders only predecessor. TODO: Could DoLoopStart get moved into the
-/// pre-preheader?
+/// preheaders only predecessor.
/// - t2LoopDec - placed within in the loop body.
/// - t2LoopEnd - the loop latch terminator.
///
@@ -176,19 +175,25 @@
// faster than performing a sub,cmp,br or even subs,br.
Revert = true;
- if (!Dec)
+ if (!Dec || End)
continue;
- // If we find that we load/store LR between LoopDec and LoopEnd, expect
- // that the decremented value has been spilled to the stack. Because
- // this value isn't actually going to be produced until the latch, by LE,
- // we would need to generate a real sub. The value is also likely to be
- // reloaded for use of LoopEnd - in which in case we'd need to perform
- // an add because it gets negated again by LE! The other option is to
- // then generate the other form of LE which doesn't perform the sub.
- if (MI.mayLoad() || MI.mayStore())
- Revert =
- MI.getOperand(0).isReg() && MI.getOperand(0).getReg() == ARM::LR;
+ // If we find that LR has been written or read between LoopDec and
+ // LoopEnd, expect that the decremented value is being used else where.
+ // Because this value isn't actually going to be produced until the
+ // latch, by LE, we would need to generate a real sub. The value is also
+ // likely to be copied/reloaded for use of LoopEnd - in which in case
+ // we'd need to perform an add because it gets subtracted again by LE!
+ // The other option is to then generate the other form of LE which doesn't
+ // perform the sub.
+ for (auto &MO : MI.operands()) {
+ if (MI.getOpcode() != ARM::t2LoopDec && MO.isReg() &&
+ MO.getReg() == ARM::LR) {
+ LLVM_DEBUG(dbgs() << "ARM Loops: Found LR Use/Def: " << MI);
+ Revert = true;
+ break;
+ }
+ }
}
if (Dec && End && Revert)