bool runOnMachineFunction(MachineFunction &MF) override;
private:
- bool shouldPerformTransformation(MachineFunction &MF);
-
// Information we know about a particular call site
struct CallContext {
CallContext()
: Call(nullptr), SPCopy(nullptr), ExpectedDist(0),
- MovVector(4, nullptr), UsePush(false){};
+ MovVector(4, nullptr), NoStackParams(false), UsePush(false){};
// Actuall call instruction
MachineInstr *Call;
// The sequence of movs used to pass the parameters
SmallVector<MachineInstr *, 4> MovVector;
- // Whether this site should use push instructions
+ // True if this call site has no stack parameters
+ bool NoStackParams;
+
+ // True of this callsite can use push instructions
bool UsePush;
};
+ typedef DenseMap<MachineInstr *, CallContext> ContextMap;
+
+ bool isLegal(MachineFunction &MF);
+
+ bool isProfitable(MachineFunction &MF, ContextMap &CallSeqMap);
+
void collectCallInfo(MachineFunction &MF, MachineBasicBlock &MBB,
MachineBasicBlock::iterator I, CallContext &Context);
return new X86CallFrameOptimization();
}
-// This checks whether the transformation is legal and profitable
-bool X86CallFrameOptimization::shouldPerformTransformation(
- MachineFunction &MF) {
+// This checks whether the transformation is legal.
+// Also returns false in cases where it's potentially legal, but
+// we don't even want to try.
+bool X86CallFrameOptimization::isLegal(MachineFunction &MF) {
if (NoX86CFOpt.getValue())
return false;
// No point in running this in 64-bit mode, since some arguments are
// passed in-register in all common calling conventions, so the pattern
// we're looking for will never match.
- const X86Subtarget &STI = MF.getTarget().getSubtarget<X86Subtarget>();
+ const X86Subtarget &STI = MF.getSubtarget<X86Subtarget>();
if (STI.is64Bit())
return false;
return false;
}
- // Now that we know the transformation is legal, check if it is
- // profitable.
- // TODO: Add a heuristic that actually looks at the function,
- // and enable this for more cases.
+ return true;
+}
- // This transformation is always a win when we expected to have
+// Check whether this trasnformation is profitable for a particular
+// function - in terms of code size.
+bool X86CallFrameOptimization::isProfitable(MachineFunction &MF,
+ ContextMap &CallSeqMap) {
+ // This transformation is always a win when we do not expect to have
// a reserved call frame. Under other circumstances, it may be either
// a win or a loss, and requires a heuristic.
- // For now, enable it only for the relatively clear win cases.
bool CannotReserveFrame = MF.getFrameInfo()->hasVarSizedObjects();
if (CannotReserveFrame)
return true;
- // For now, don't even try to evaluate the profitability when
- // not optimizing for size.
- AttributeSet FnAttrs = MF.getFunction()->getAttributes();
+ // Don't do this when not optimizing for size.
bool OptForSize =
- FnAttrs.hasAttribute(AttributeSet::FunctionIndex,
- Attribute::OptimizeForSize) ||
- FnAttrs.hasAttribute(AttributeSet::FunctionIndex, Attribute::MinSize);
+ MF.getFunction()->hasFnAttribute(Attribute::OptimizeForSize) ||
+ MF.getFunction()->hasFnAttribute(Attribute::MinSize);
if (!OptForSize)
return false;
- // Stack re-alignment can make this unprofitable even in terms of size.
- // As mentioned above, a better heuristic is needed. For now, don't do this
- // when the required alignment is above 8. (4 would be the safe choice, but
- // some experimentation showed 8 is generally good).
- if (TFL->getStackAlignment() > 8)
- return false;
- return true;
+ unsigned StackAlign = TFL->getStackAlignment();
+
+ int64_t Advantage = 0;
+ for (auto CC : CallSeqMap) {
+ // Call sites where no parameters are passed on the stack
+ // do not affect the cost, since there needs to be no
+ // stack adjustment.
+ if (CC.second.NoStackParams)
+ continue;
+
+ if (!CC.second.UsePush) {
+ // If we don't use pushes for a particular call site,
+ // we pay for not having a reserved call frame with an
+ // additional sub/add esp pair. The cost is ~3 bytes per instruction,
+ // depending on the size of the constant.
+ // TODO: Callee-pop functions should have a smaller penalty, because
+ // an add is needed even with a reserved call frame.
+ Advantage -= 6;
+ } else {
+ // We can use pushes. First, account for the fixed costs.
+ // We'll need a add after the call.
+ Advantage -= 3;
+ // If we have to realign the stack, we'll also need and sub before
+ if (CC.second.ExpectedDist % StackAlign)
+ Advantage -= 3;
+ // Now, for each push, we save ~3 bytes. For small constants, we actually,
+ // save more (up to 5 bytes), but 3 should be a good approximation.
+ Advantage += (CC.second.ExpectedDist / 4) * 3;
+ }
+ }
+
+ return (Advantage >= 0);
}
+
bool X86CallFrameOptimization::runOnMachineFunction(MachineFunction &MF) {
TII = MF.getSubtarget().getInstrInfo();
TFL = MF.getSubtarget().getFrameLowering();
MRI = &MF.getRegInfo();
- if (!shouldPerformTransformation(MF))
+ if (!isLegal(MF))
return false;
int FrameSetupOpcode = TII->getCallFrameSetupOpcode();
bool Changed = false;
- DenseMap<MachineInstr *, CallContext> CallSeqMap;
+ ContextMap CallSeqMap;
for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); BB != E; ++BB)
for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ++I)
collectCallInfo(MF, *BB, I, Context);
}
+ if (!isProfitable(MF, CallSeqMap))
+ return false;
+
for (auto CC : CallSeqMap)
if (CC.second.UsePush)
Changed |= adjustCallSequence(MF, CC.first, CC.second);
assert(I->getOpcode() == TII->getCallFrameSetupOpcode());
MachineBasicBlock::iterator FrameSetup = I++;
+ // How much do we adjust the stack? This puts an upper bound on
+ // the number of parameters actually passed on it.
+ unsigned int MaxAdjust = FrameSetup->getOperand(0).getImm() / 4;
+
+ // A zero adjustment means no stack parameters
+ if (!MaxAdjust) {
+ Context.NoStackParams = true;
+ return;
+ }
+
// For globals in PIC mode, we can have some LEAs here.
// Ignore them, they don't bother us.
// TODO: Extend this to something that covers more cases.
// We only handle a simple case - a sequence of MOV32mi or MOV32mr
// instructions, that push a sequence of 32-bit values onto the stack, with
// no gaps between them.
- unsigned int MaxAdjust = FrameSetup->getOperand(0).getImm() / 4;
if (MaxAdjust > 4)
Context.MovVector.resize(MaxAdjust, nullptr);
// If PUSHrmm is not slow on this target, try to fold the source of the
// push into the instruction.
- const X86Subtarget &ST = MF.getTarget().getSubtarget<X86Subtarget>();
+ const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();
bool SlowPUSHrmm = ST.isAtom() || ST.isSLM();
// Check that this is legal to fold. Right now, we're extremely
DefMI->getParent() != FrameSetup->getParent())
return nullptr;
- // Be careful with movs that load from a stack slot, since it may get
- // resolved incorrectly.
- // TODO: Again, we already have the infrastructure, so this should work.
- if (!DefMI->getOperand(1).isReg())
- return nullptr;
-
// Now, make sure everything else up until the ADJCALLSTACK is a sequence
// of MOVs. To be less conservative would require duplicating a lot of the
// logic from PeepholeOptimizer.
projects / oota-llvm.git / blobdiff