--- /dev/null
+//===-- SILowerFlowControl.cpp - Use predicates for flow control ----------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass lowers the pseudo flow control instructions (SI_IF_NZ, ELSE, ENDIF)
+// to predicated instructions.
+//
+// All flow control (except loops) is handled using predicated instructions and
+// a predicate stack. Each Scalar ALU controls the operations of 64 Vector
+// ALUs. The Scalar ALU can update the predicate for any of the Vector ALUs
+// by writting to the 64-bit EXEC register (each bit corresponds to a
+// single vector ALU). Typically, for predicates, a vector ALU will write
+// to its bit of the VCC register (like EXEC VCC is 64-bits, one for each
+// Vector ALU) and then the ScalarALU will AND the VCC register with the
+// EXEC to update the predicates.
+//
+// For example:
+// %VCC = V_CMP_GT_F32 %VGPR1, %VGPR2
+// SI_IF_NZ %VCC
+// %VGPR0 = V_ADD_F32 %VGPR0, %VGPR0
+// ELSE
+// %VGPR0 = V_SUB_F32 %VGPR0, %VGPR0
+// ENDIF
+//
+// becomes:
+//
+// %SGPR0 = S_MOV_B64 %EXEC // Save the current exec mask
+// %EXEC = S_AND_B64 %VCC, %EXEC // Update the exec mask
+// S_CBRANCH_EXECZ label0 // This instruction is an
+// // optimization which allows us to
+// // branch if all the bits of
+// // EXEC are zero.
+// %VGPR0 = V_ADD_F32 %VGPR0, %VGPR0 // Do the IF block of the branch
+//
+// label0:
+// %EXEC = S_NOT_B64 %EXEC // Invert the exec mask for the
+// // Then block.
+// %EXEC = S_AND_B64 %SGPR0, %EXEC
+// S_BRANCH_EXECZ label1 // Use our branch optimization
+// // instruction again.
+// %VGPR0 = V_SUB_F32 %VGPR0, %VGPR // Do the THEN block
+// label1:
+// S_MOV_B64 // Restore the old EXEC value
+//===----------------------------------------------------------------------===//
+
+#include "AMDGPU.h"
+#include "SIInstrInfo.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+
+using namespace llvm;
+
+namespace {
+
+class SILowerFlowControlPass : public MachineFunctionPass {
+
+private:
+ static char ID;
+ const TargetInstrInfo *TII;
+ std::vector<unsigned> PredicateStack;
+ std::vector<unsigned> UnusedRegisters;
+
+ void pushExecMask(MachineBasicBlock &MBB, MachineBasicBlock::iterator I);
+ void popExecMask(MachineBasicBlock &MBB, MachineBasicBlock::iterator I);
+
+public:
+ SILowerFlowControlPass(TargetMachine &tm) :
+ MachineFunctionPass(ID), TII(tm.getInstrInfo()) { }
+
+ virtual bool runOnMachineFunction(MachineFunction &MF);
+
+ const char *getPassName() const {
+ return "SI Lower flow control instructions";
+ }
+
+};
+
+} // End anonymous namespace
+
+char SILowerFlowControlPass::ID = 0;
+
+FunctionPass *llvm::createSILowerFlowControlPass(TargetMachine &tm) {
+ return new SILowerFlowControlPass(tm);
+}
+
+bool SILowerFlowControlPass::runOnMachineFunction(MachineFunction &MF) {
+
+ // Find all the unused registers that can be used for the predicate stack.
+ for (TargetRegisterClass::iterator S = AMDGPU::SReg_64RegClass.begin(),
+ I = AMDGPU::SReg_64RegClass.end();
+ I != S; --I) {
+ unsigned Reg = *I;
+ if (!MF.getRegInfo().isPhysRegOrOverlapUsed(Reg)) {
+ UnusedRegisters.push_back(Reg);
+ }
+ }
+
+ for (MachineFunction::iterator BB = MF.begin(), BB_E = MF.end();
+ BB != BB_E; ++BB) {
+ MachineBasicBlock &MBB = *BB;
+ for (MachineBasicBlock::iterator I = MBB.begin(), Next = llvm::next(I);
+ I != MBB.end(); I = Next, Next = llvm::next(I)) {
+ MachineInstr &MI = *I;
+ switch (MI.getOpcode()) {
+ default: break;
+ case AMDGPU::SI_IF_NZ:
+ pushExecMask(MBB, I);
+ BuildMI(MBB, I, MBB.findDebugLoc(I), TII->get(AMDGPU::S_AND_B64),
+ AMDGPU::EXEC)
+ .addOperand(MI.getOperand(0)) // VCC
+ .addReg(AMDGPU::EXEC);
+ MI.eraseFromParent();
+ break;
+ case AMDGPU::ELSE:
+ BuildMI(MBB, I, MBB.findDebugLoc(I), TII->get(AMDGPU::S_NOT_B64),
+ AMDGPU::EXEC)
+ .addReg(AMDGPU::EXEC);
+ BuildMI(MBB, I, MBB.findDebugLoc(I), TII->get(AMDGPU::S_AND_B64),
+ AMDGPU::EXEC)
+ .addReg(PredicateStack.back())
+ .addReg(AMDGPU::EXEC);
+ MI.eraseFromParent();
+ break;
+ case AMDGPU::ENDIF:
+ popExecMask(MBB, I);
+ MI.eraseFromParent();
+ break;
+ }
+ }
+ }
+ return false;
+}
+
+void SILowerFlowControlPass::pushExecMask(MachineBasicBlock &MBB,
+ MachineBasicBlock::iterator I) {
+
+ assert(!UnusedRegisters.empty() && "Ran out of registers for predicate stack");
+ unsigned StackReg = UnusedRegisters.back();
+ UnusedRegisters.pop_back();
+ PredicateStack.push_back(StackReg);
+ BuildMI(MBB, I, MBB.findDebugLoc(I), TII->get(AMDGPU::S_MOV_B64),
+ StackReg)
+ .addReg(AMDGPU::EXEC);
+}
+
+void SILowerFlowControlPass::popExecMask(MachineBasicBlock &MBB,
+ MachineBasicBlock::iterator I) {
+ unsigned StackReg = PredicateStack.back();
+ PredicateStack.pop_back();
+ UnusedRegisters.push_back(StackReg);
+ BuildMI(MBB, I, MBB.findDebugLoc(I), TII->get(AMDGPU::S_MOV_B64),
+ AMDGPU::EXEC)
+ .addReg(StackReg);
+}
projects / mesa.git / commitdiff