#define DEBUG_TYPE "execution-fix"
#include "llvm/CodeGen/Passes.h"
#include "llvm/ADT/PostOrderIterator.h"
+#include "llvm/CodeGen/LiveRegUnits.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Support/Allocator.h"
typedef DenseMap<MachineBasicBlock*, LiveReg*> LiveOutMap;
LiveOutMap LiveOuts;
+ /// List of undefined register reads in this block in forward order.
+ std::vector<std::pair<MachineInstr*, unsigned> > UndefReads;
+
+ /// Storage for register unit liveness.
+ LiveRegUnits LiveUnits;
+
/// Current instruction number.
/// The first instruction in each basic block is 0.
int CurInstr;
void processDefs(MachineInstr*, bool Kill);
void visitSoftInstr(MachineInstr*, unsigned mask);
void visitHardInstr(MachineInstr*, unsigned domain);
+ bool shouldBreakDependence(MachineInstr*, unsigned OpIdx, unsigned Pref);
+ void processUndefReads(MachineBasicBlock*);
};
}
// Reset instruction counter in each basic block.
CurInstr = 0;
+ // Set up UndefReads to track undefined register reads.
+ UndefReads.clear();
+ LiveUnits.clear();
+
// Set up LiveRegs to represent registers entering MBB.
if (!LiveRegs)
LiveRegs = new LiveReg[NumRegs];
processDefs(MI, !DomP.first);
}
+/// \brief Return true to if it makes sense to break dependence on a partial def
+/// or undef use.
+bool ExeDepsFix::shouldBreakDependence(MachineInstr *MI, unsigned OpIdx,
+ unsigned Pref) {
+ int rx = regIndex(MI->getOperand(OpIdx).getReg());
+ if (rx < 0)
+ return false;
+
+ unsigned Clearance = CurInstr - LiveRegs[rx].Def;
+ DEBUG(dbgs() << "Clearance: " << Clearance << ", want " << Pref);
+
+ if (Pref > Clearance) {
+ DEBUG(dbgs() << ": Break dependency.\n");
+ return true;
+ }
+ // The current clearance seems OK, but we may be ignoring a def from a
+ // back-edge.
+ if (!SeenUnknownBackEdge || Pref <= unsigned(CurInstr)) {
+ DEBUG(dbgs() << ": OK .\n");
+ return false;
+ }
+ // A def from an unprocessed back-edge may make us break this dependency.
+ DEBUG(dbgs() << ": Wait for back-edge to resolve.\n");
+ return false;
+}
+
// Update def-ages for registers defined by MI.
// If Kill is set, also kill off DomainValues clobbered by the defs.
+//
+// Also break dependencies on partial defs and undef uses.
void ExeDepsFix::processDefs(MachineInstr *MI, bool Kill) {
assert(!MI->isDebugValue() && "Won't process debug values");
+
+ // Break dependence on undef uses. Do this before updating LiveRegs below.
+ unsigned OpNum;
+ unsigned Pref = TII->getUndefRegClearance(MI, OpNum, TRI);
+ if (Pref) {
+ if (shouldBreakDependence(MI, OpNum, Pref))
+ UndefReads.push_back(std::make_pair(MI, OpNum));
+ }
const MCInstrDesc &MCID = MI->getDesc();
for (unsigned i = 0,
e = MI->isVariadic() ? MI->getNumOperands() : MCID.getNumDefs();
DEBUG(dbgs() << TRI->getName(RC->getRegister(rx)) << ":\t" << CurInstr
<< '\t' << *MI);
+ // Check clearance before partial register updates.
+ // Call breakDependence before setting LiveRegs[rx].Def.
+ unsigned Pref = TII->getPartialRegUpdateClearance(MI, i, TRI);
+ if (Pref && shouldBreakDependence(MI, i, Pref))
+ TII->breakPartialRegDependency(MI, i, TRI);
+
// How many instructions since rx was last written?
- unsigned Clearance = CurInstr - LiveRegs[rx].Def;
LiveRegs[rx].Def = CurInstr;
// Kill off domains redefined by generic instructions.
if (Kill)
kill(rx);
+ }
+ ++CurInstr;
+}
- // Verify clearance before partial register updates.
- unsigned Pref = TII->getPartialRegUpdateClearance(MI, i, TRI);
- if (!Pref)
- continue;
- DEBUG(dbgs() << "Clearance: " << Clearance << ", want " << Pref);
- if (Pref > Clearance) {
- DEBUG(dbgs() << ": Break dependency.\n");
- TII->breakPartialRegDependency(MI, i, TRI);
- continue;
- }
-
- // The current clearance seems OK, but we may be ignoring a def from a
- // back-edge.
- if (!SeenUnknownBackEdge || Pref <= unsigned(CurInstr)) {
- DEBUG(dbgs() << ": OK.\n");
- continue;
- }
+/// \break Break false dependencies on undefined register reads.
+///
+/// Walk the block backward computing precise liveness. This is expensive, so we
+/// only do it on demand. Note that the occurrence of undefined register reads
+/// that should be broken is very rare, but when they occur we may have many in
+/// a single block.
+void ExeDepsFix::processUndefReads(MachineBasicBlock *MBB) {
+ if (UndefReads.empty())
+ return;
- // A def from an unprocessed back-edge may make us break this dependency.
- DEBUG(dbgs() << ": Wait for back-edge to resolve.\n");
+ // Collect this block's live out register units.
+ LiveUnits.init(TRI);
+ for (MachineBasicBlock::const_succ_iterator SI = MBB->succ_begin(),
+ SE = MBB->succ_end(); SI != SE; ++SI) {
+ LiveUnits.addLiveIns(*SI, *TRI);
}
+ MachineInstr *UndefMI = UndefReads.back().first;
+ unsigned OpIdx = UndefReads.back().second;
- ++CurInstr;
+ for (MachineBasicBlock::reverse_iterator I = MBB->rbegin(), E = MBB->rend();
+ I != E; ++I) {
+ if (UndefMI == &*I) {
+ if (!LiveUnits.contains(UndefMI->getOperand(OpIdx).getReg(), *TRI))
+ TII->breakPartialRegDependency(UndefMI, OpIdx, TRI);
+
+ UndefReads.pop_back();
+ if (UndefReads.empty())
+ return;
+
+ UndefMI = UndefReads.back().first;
+ OpIdx = UndefReads.back().second;
+ }
+ LiveUnits.stepBackward(*I, *TRI);
+ }
}
// A hard instruction only works in one domain. All input registers will be
// Is it possible to use this collapsed register for free?
if (dv->isCollapsed()) {
// Restrict available domains to the ones in common with the operand.
- // If there are no common domains, we must pay the cross-domain
+ // If there are no common domains, we must pay the cross-domain
// penalty for this operand.
if (common) available = common;
} else if (common)
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E;
++I)
visitInstr(I);
+ processUndefReads(MBB);
leaveBasicBlock(MBB);
}
++I)
if (!I->isDebugValue())
processDefs(I, false);
+ processUndefReads(MBB);
leaveBasicBlock(MBB);
}
delete[] FI->second;
}
LiveOuts.clear();
+ UndefReads.clear();
Avail.clear();
Allocator.DestroyAll();
case X86::RSQRTSSr_Int:
case X86::SQRTSSr:
case X86::SQRTSSr_Int:
- // AVX encoded versions
- case X86::VCVTSD2SSrr:
- case X86::Int_VCVTSD2SSrr:
- case X86::VCVTSS2SDrr:
- case X86::Int_VCVTSS2SDrr:
- case X86::VCVTSD2SSZrr:
- case X86::VCVTSS2SDZrr:
- case X86::VRCPSSr:
- case X86::VROUNDSDr:
- case X86::VROUNDSDr_Int:
- case X86::VROUNDSSr:
- case X86::VROUNDSSr_Int:
- case X86::VRSQRTSSr:
- case X86::VSQRTSSr:
return true;
}
return 16;
}
+// Return true for any instruction the copies the high bits of the first source
+// operand into the unused high bits of the destination operand.
+static bool hasUndefRegUpdate(unsigned Opcode) {
+ switch (Opcode) {
+ case X86::VCVTSI2SSrr:
+ case X86::Int_VCVTSI2SSrr:
+ case X86::VCVTSI2SS64rr:
+ case X86::Int_VCVTSI2SS64rr:
+ case X86::VCVTSI2SDrr:
+ case X86::Int_VCVTSI2SDrr:
+ case X86::VCVTSI2SD64rr:
+ case X86::Int_VCVTSI2SD64rr:
+ case X86::VCVTSD2SSrr:
+ case X86::Int_VCVTSD2SSrr:
+ case X86::VCVTSS2SDrr:
+ case X86::Int_VCVTSS2SDrr:
+ case X86::VRCPSSr:
+ case X86::VROUNDSDr:
+ case X86::VROUNDSDr_Int:
+ case X86::VROUNDSSr:
+ case X86::VROUNDSSr_Int:
+ case X86::VRSQRTSSr:
+ case X86::VSQRTSSr:
+
+ // AVX-512
+ case X86::VCVTSD2SSZrr:
+ case X86::VCVTSS2SDZrr:
+ return true;
+ }
+
+ return false;
+}
+
+/// Inform the ExeDepsFix pass how many idle instructions we would like before
+/// certain undef register reads.
+///
+/// This catches the VCVTSI2SD family of instructions:
+///
+/// vcvtsi2sdq %rax, %xmm0<undef>, %xmm14
+///
+/// We should to be careful *not* to catch VXOR idioms which are presumably
+/// handled specially in the pipeline:
+///
+/// vxorps %xmm1<undef>, %xmm1<undef>, %xmm1
+///
+/// Like getPartialRegUpdateClearance, this makes a strong assumption that the
+/// high bits that are passed-through are not live.
+unsigned X86InstrInfo::
+getUndefRegClearance(const MachineInstr *MI, unsigned &OpNum,
+ const TargetRegisterInfo *TRI) const {
+ if (!hasUndefRegUpdate(MI->getOpcode()))
+ return 0;
+
+ // Set the OpNum parameter to the first source operand.
+ OpNum = 1;
+
+ const MachineOperand &MO = MI->getOperand(OpNum);
+ if (MO.isUndef() && TargetRegisterInfo::isPhysicalRegister(MO.getReg())) {
+ // Use the same magic number as getPartialRegUpdateClearance.
+ return 16;
+ }
+ return 0;
+}
+
void X86InstrInfo::
breakPartialRegDependency(MachineBasicBlock::iterator MI, unsigned OpNum,
const TargetRegisterInfo *TRI) const {
unsigned Reg = MI->getOperand(OpNum).getReg();
+ // If MI kills this register, the false dependence is already broken.
+ if (MI->killsRegister(Reg, TRI))
+ return;
if (X86::VR128RegClass.contains(Reg)) {
// These instructions are all floating point domain, so xorps is the best
// choice.
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