1 //===-- RISCVMakeCompressible.cpp - Make more instructions compressible ---===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This pass searches for instructions that are prevented from being compressed
10 // by one of the following:
11 //
12 // 1. The use of a single uncompressed register.
13 // 2. A base register + offset where the offset is too large to be compressed
14 // and the base register may or may not be compressed.
15 //
16 //
17 // For case 1, if a compressed register is available, then the uncompressed
18 // register is copied to the compressed register and its uses are replaced.
19 //
20 // For example, storing zero uses the uncompressible zero register:
21 // sw zero, 0(a0) # if zero
22 // sw zero, 8(a0) # if zero
23 // sw zero, 4(a0) # if zero
24 // sw zero, 24(a0) # if zero
25 //
26 // If a compressed register (e.g. a1) is available, the above can be transformed
27 // to the following to improve code size:
28 // li a1, 0
29 // c.sw a1, 0(a0)
30 // c.sw a1, 8(a0)
31 // c.sw a1, 4(a0)
32 // c.sw a1, 24(a0)
33 //
34 //
35 // For case 2, if a compressed register is available, then the original base
36 // is copied and adjusted such that:
37 //
38 // new_base_register = base_register + adjustment
39 // base_register + large_offset = new_base_register + small_offset
40 //
41 // For example, the following offsets are too large for c.sw:
42 // lui a2, 983065
43 // sw a1, -236(a2)
44 // sw a1, -240(a2)
45 // sw a1, -244(a2)
46 // sw a1, -248(a2)
47 // sw a1, -252(a2)
48 // sw a0, -256(a2)
49 //
50 // If a compressed register is available (e.g. a3), a new base could be created
51 // such that the addresses can accessed with a compressible offset, thus
52 // improving code size:
53 // lui a2, 983065
54 // addi a3, a2, -256
55 // c.sw a1, 20(a3)
56 // c.sw a1, 16(a3)
57 // c.sw a1, 12(a3)
58 // c.sw a1, 8(a3)
59 // c.sw a1, 4(a3)
60 // c.sw a0, 0(a3)
61 //
62 //
63 // This optimization is only applied if there are enough uses of the copied
64 // register for code size to be reduced.
65 //
66 //===----------------------------------------------------------------------===//
67
68 #include "RISCV.h"
69 #include "RISCVSubtarget.h"
70 #include "llvm/CodeGen/Passes.h"
71 #include "llvm/CodeGen/RegisterScavenging.h"
72 #include "llvm/MC/TargetRegistry.h"
73 #include "llvm/Support/Debug.h"
74
75 using namespace llvm;
76
77 #define DEBUG_TYPE "riscv-make-compressible"
78 #define RISCV_COMPRESS_INSTRS_NAME "RISC-V Make Compressible"
79
80 namespace {
81
82 struct RISCVMakeCompressibleOpt : public MachineFunctionPass {
83 static char ID;
84
85 bool runOnMachineFunction(MachineFunction &Fn) override;
86
RISCVMakeCompressibleOpt__anonac32bc890111::RISCVMakeCompressibleOpt87 RISCVMakeCompressibleOpt() : MachineFunctionPass(ID) {}
88
getPassName__anonac32bc890111::RISCVMakeCompressibleOpt89 StringRef getPassName() const override { return RISCV_COMPRESS_INSTRS_NAME; }
90 };
91 } // namespace
92
93 char RISCVMakeCompressibleOpt::ID = 0;
94 INITIALIZE_PASS(RISCVMakeCompressibleOpt, "riscv-make-compressible",
95 RISCV_COMPRESS_INSTRS_NAME, false, false)
96
97 // Return log2(widthInBytes) of load/store done by Opcode.
log2LdstWidth(unsigned Opcode)98 static unsigned log2LdstWidth(unsigned Opcode) {
99 switch (Opcode) {
100 default:
101 llvm_unreachable("Unexpected opcode");
102 case RISCV::LBU:
103 case RISCV::SB:
104 return 0;
105 case RISCV::LH:
106 case RISCV::LHU:
107 case RISCV::SH:
108 return 1;
109 case RISCV::LW:
110 case RISCV::SW:
111 case RISCV::FLW:
112 case RISCV::FSW:
113 return 2;
114 case RISCV::LD:
115 case RISCV::SD:
116 case RISCV::FLD:
117 case RISCV::FSD:
118 return 3;
119 }
120 }
121
122 // Return bit field size of immediate operand of Opcode.
offsetMask(unsigned Opcode)123 static unsigned offsetMask(unsigned Opcode) {
124 switch (Opcode) {
125 default:
126 llvm_unreachable("Unexpected opcode");
127 case RISCV::LBU:
128 case RISCV::SB:
129 return maskTrailingOnes<unsigned>(2U);
130 case RISCV::LH:
131 case RISCV::LHU:
132 case RISCV::SH:
133 return maskTrailingOnes<unsigned>(1U);
134 case RISCV::LW:
135 case RISCV::SW:
136 case RISCV::FLW:
137 case RISCV::FSW:
138 case RISCV::LD:
139 case RISCV::SD:
140 case RISCV::FLD:
141 case RISCV::FSD:
142 return maskTrailingOnes<unsigned>(5U);
143 }
144 }
145
146 // Return a mask for the offset bits of a non-stack-pointer based compressed
147 // load/store.
compressedLDSTOffsetMask(unsigned Opcode)148 static uint8_t compressedLDSTOffsetMask(unsigned Opcode) {
149 return offsetMask(Opcode) << log2LdstWidth(Opcode);
150 }
151
152 // Return true if Offset fits within a compressed stack-pointer based
153 // load/store.
compressibleSPOffset(int64_t Offset,unsigned Opcode)154 static bool compressibleSPOffset(int64_t Offset, unsigned Opcode) {
155 // Compressed sp-based loads and stores only work for 32/64 bits.
156 switch (log2LdstWidth(Opcode)) {
157 case 2:
158 return isShiftedUInt<6, 2>(Offset);
159 case 3:
160 return isShiftedUInt<6, 3>(Offset);
161 }
162 return false;
163 }
164
165 // Given an offset for a load/store, return the adjustment required to the base
166 // register such that the address can be accessed with a compressible offset.
167 // This will return 0 if the offset is already compressible.
getBaseAdjustForCompression(int64_t Offset,unsigned Opcode)168 static int64_t getBaseAdjustForCompression(int64_t Offset, unsigned Opcode) {
169 // Return the excess bits that do not fit in a compressible offset.
170 return Offset & ~compressedLDSTOffsetMask(Opcode);
171 }
172
173 // Return true if Reg is in a compressed register class.
isCompressedReg(Register Reg)174 static bool isCompressedReg(Register Reg) {
175 return RISCV::GPRCRegClass.contains(Reg) ||
176 RISCV::FPR32CRegClass.contains(Reg) ||
177 RISCV::FPR64CRegClass.contains(Reg);
178 }
179
180 // Return true if MI is a load for which there exists a compressed version.
isCompressibleLoad(const MachineInstr & MI)181 static bool isCompressibleLoad(const MachineInstr &MI) {
182 const RISCVSubtarget &STI = MI.getMF()->getSubtarget<RISCVSubtarget>();
183
184 switch (MI.getOpcode()) {
185 default:
186 return false;
187 case RISCV::LBU:
188 case RISCV::LH:
189 case RISCV::LHU:
190 return STI.hasStdExtZcb();
191 case RISCV::LW:
192 case RISCV::LD:
193 return STI.hasStdExtCOrZca();
194 case RISCV::FLW:
195 return !STI.is64Bit() && STI.hasStdExtCOrZcfOrZce();
196 case RISCV::FLD:
197 return STI.hasStdExtCOrZcd();
198 }
199 }
200
201 // Return true if MI is a store for which there exists a compressed version.
isCompressibleStore(const MachineInstr & MI)202 static bool isCompressibleStore(const MachineInstr &MI) {
203 const RISCVSubtarget &STI = MI.getMF()->getSubtarget<RISCVSubtarget>();
204
205 switch (MI.getOpcode()) {
206 default:
207 return false;
208 case RISCV::SB:
209 case RISCV::SH:
210 return STI.hasStdExtZcb();
211 case RISCV::SW:
212 case RISCV::SD:
213 return STI.hasStdExtCOrZca();
214 case RISCV::FSW:
215 return !STI.is64Bit() && STI.hasStdExtCOrZcfOrZce();
216 case RISCV::FSD:
217 return STI.hasStdExtCOrZcd();
218 }
219 }
220
221 // Find a single register and/or large offset which, if compressible, would
222 // allow the given instruction to be compressed.
223 //
224 // Possible return values:
225 //
226 // {Reg, 0} - Uncompressed Reg needs replacing with a compressed
227 // register.
228 // {Reg, N} - Reg needs replacing with a compressed register and
229 // N needs adding to the new register. (Reg may be
230 // compressed or uncompressed).
231 // {RISCV::NoRegister, 0} - No suitable optimization found for this
232 // instruction.
getRegImmPairPreventingCompression(const MachineInstr & MI)233 static RegImmPair getRegImmPairPreventingCompression(const MachineInstr &MI) {
234 const unsigned Opcode = MI.getOpcode();
235
236 if (isCompressibleLoad(MI) || isCompressibleStore(MI)) {
237 const MachineOperand &MOImm = MI.getOperand(2);
238 if (!MOImm.isImm())
239 return RegImmPair(RISCV::NoRegister, 0);
240
241 int64_t Offset = MOImm.getImm();
242 int64_t NewBaseAdjust = getBaseAdjustForCompression(Offset, Opcode);
243 Register Base = MI.getOperand(1).getReg();
244
245 // Memory accesses via the stack pointer do not have a requirement for
246 // either of the registers to be compressible and can take a larger offset.
247 if (RISCV::SPRegClass.contains(Base)) {
248 if (!compressibleSPOffset(Offset, Opcode) && NewBaseAdjust)
249 return RegImmPair(Base, NewBaseAdjust);
250 } else {
251 Register SrcDest = MI.getOperand(0).getReg();
252 bool SrcDestCompressed = isCompressedReg(SrcDest);
253 bool BaseCompressed = isCompressedReg(Base);
254
255 // If only Base and/or offset prevent compression, then return Base and
256 // any adjustment required to make the offset compressible.
257 if ((!BaseCompressed || NewBaseAdjust) && SrcDestCompressed)
258 return RegImmPair(Base, NewBaseAdjust);
259
260 // For loads, we can only change the base register since dest is defined
261 // rather than used.
262 //
263 // For stores, we can change SrcDest (and Base if SrcDest == Base) but
264 // cannot resolve an uncompressible offset in this case.
265 if (isCompressibleStore(MI)) {
266 if (!SrcDestCompressed && (BaseCompressed || SrcDest == Base) &&
267 !NewBaseAdjust)
268 return RegImmPair(SrcDest, NewBaseAdjust);
269 }
270 }
271 }
272 return RegImmPair(RISCV::NoRegister, 0);
273 }
274
275 // Check all uses after FirstMI of the given register, keeping a vector of
276 // instructions that would be compressible if the given register (and offset if
277 // applicable) were compressible.
278 //
279 // If there are enough uses for this optimization to improve code size and a
280 // compressed register is available, return that compressed register.
analyzeCompressibleUses(MachineInstr & FirstMI,RegImmPair RegImm,SmallVectorImpl<MachineInstr * > & MIs)281 static Register analyzeCompressibleUses(MachineInstr &FirstMI,
282 RegImmPair RegImm,
283 SmallVectorImpl<MachineInstr *> &MIs) {
284 MachineBasicBlock &MBB = *FirstMI.getParent();
285 const TargetRegisterInfo *TRI =
286 MBB.getParent()->getSubtarget().getRegisterInfo();
287
288 for (MachineBasicBlock::instr_iterator I = FirstMI.getIterator(),
289 E = MBB.instr_end();
290 I != E; ++I) {
291 MachineInstr &MI = *I;
292
293 // Determine if this is an instruction which would benefit from using the
294 // new register.
295 RegImmPair CandidateRegImm = getRegImmPairPreventingCompression(MI);
296 if (CandidateRegImm.Reg == RegImm.Reg && CandidateRegImm.Imm == RegImm.Imm)
297 MIs.push_back(&MI);
298
299 // If RegImm.Reg is modified by this instruction, then we cannot optimize
300 // past this instruction. If the register is already compressed, then it may
301 // possible to optimize a large offset in the current instruction - this
302 // will have been detected by the preceeding call to
303 // getRegImmPairPreventingCompression.
304 if (MI.modifiesRegister(RegImm.Reg, TRI))
305 break;
306 }
307
308 // Adjusting the base costs one new uncompressed addi and therefore three uses
309 // are required for a code size reduction. If no base adjustment is required,
310 // then copying the register costs one new c.mv (or c.li Rd, 0 for "copying"
311 // the zero register) and therefore two uses are required for a code size
312 // reduction.
313 if (MIs.size() < 2 || (RegImm.Imm != 0 && MIs.size() < 3))
314 return RISCV::NoRegister;
315
316 // Find a compressible register which will be available from the first
317 // instruction we care about to the last.
318 const TargetRegisterClass *RCToScavenge;
319
320 // Work out the compressed register class from which to scavenge.
321 if (RISCV::GPRRegClass.contains(RegImm.Reg))
322 RCToScavenge = &RISCV::GPRCRegClass;
323 else if (RISCV::FPR32RegClass.contains(RegImm.Reg))
324 RCToScavenge = &RISCV::FPR32CRegClass;
325 else if (RISCV::FPR64RegClass.contains(RegImm.Reg))
326 RCToScavenge = &RISCV::FPR64CRegClass;
327 else
328 return RISCV::NoRegister;
329
330 RegScavenger RS;
331 RS.enterBasicBlockEnd(MBB);
332 RS.backward(std::next(MIs.back()->getIterator()));
333 return RS.scavengeRegisterBackwards(*RCToScavenge, FirstMI.getIterator(),
334 /*RestoreAfter=*/false, /*SPAdj=*/0,
335 /*AllowSpill=*/false);
336 }
337
338 // Update uses of the old register in the given instruction to the new register.
updateOperands(MachineInstr & MI,RegImmPair OldRegImm,Register NewReg)339 static void updateOperands(MachineInstr &MI, RegImmPair OldRegImm,
340 Register NewReg) {
341 unsigned Opcode = MI.getOpcode();
342
343 // If this pass is extended to support more instructions, the check for
344 // definedness may need to be strengthened.
345 assert((isCompressibleLoad(MI) || isCompressibleStore(MI)) &&
346 "Unsupported instruction for this optimization.");
347
348 int SkipN = 0;
349
350 // Skip the first (value) operand to a store instruction (except if the store
351 // offset is zero) in order to avoid an incorrect transformation.
352 // e.g. sd a0, 808(a0) to addi a2, a0, 768; sd a2, 40(a2)
353 if (isCompressibleStore(MI) && OldRegImm.Imm != 0)
354 SkipN = 1;
355
356 // Update registers
357 for (MachineOperand &MO : drop_begin(MI.operands(), SkipN))
358 if (MO.isReg() && MO.getReg() == OldRegImm.Reg) {
359 // Do not update operands that define the old register.
360 //
361 // The new register was scavenged for the range of instructions that are
362 // being updated, therefore it should not be defined within this range
363 // except possibly in the final instruction.
364 if (MO.isDef()) {
365 assert(isCompressibleLoad(MI));
366 continue;
367 }
368 // Update reg
369 MO.setReg(NewReg);
370 }
371
372 // Update offset
373 MachineOperand &MOImm = MI.getOperand(2);
374 int64_t NewOffset = MOImm.getImm() & compressedLDSTOffsetMask(Opcode);
375 MOImm.setImm(NewOffset);
376 }
377
runOnMachineFunction(MachineFunction & Fn)378 bool RISCVMakeCompressibleOpt::runOnMachineFunction(MachineFunction &Fn) {
379 // This is a size optimization.
380 if (skipFunction(Fn.getFunction()) || !Fn.getFunction().hasMinSize())
381 return false;
382
383 const RISCVSubtarget &STI = Fn.getSubtarget<RISCVSubtarget>();
384 const RISCVInstrInfo &TII = *STI.getInstrInfo();
385
386 // This optimization only makes sense if compressed instructions are emitted.
387 if (!STI.hasStdExtCOrZca())
388 return false;
389
390 for (MachineBasicBlock &MBB : Fn) {
391 LLVM_DEBUG(dbgs() << "MBB: " << MBB.getName() << "\n");
392 for (MachineInstr &MI : MBB) {
393 // Determine if this instruction would otherwise be compressed if not for
394 // an uncompressible register or offset.
395 RegImmPair RegImm = getRegImmPairPreventingCompression(MI);
396 if (!RegImm.Reg && RegImm.Imm == 0)
397 continue;
398
399 // Determine if there is a set of instructions for which replacing this
400 // register with a compressed register (and compressible offset if
401 // applicable) is possible and will allow compression.
402 SmallVector<MachineInstr *, 8> MIs;
403 Register NewReg = analyzeCompressibleUses(MI, RegImm, MIs);
404 if (!NewReg)
405 continue;
406
407 // Create the appropriate copy and/or offset.
408 if (RISCV::GPRRegClass.contains(RegImm.Reg)) {
409 assert(isInt<12>(RegImm.Imm));
410 BuildMI(MBB, MI, MI.getDebugLoc(), TII.get(RISCV::ADDI), NewReg)
411 .addReg(RegImm.Reg)
412 .addImm(RegImm.Imm);
413 } else {
414 // If we are looking at replacing an FPR register we don't expect to
415 // have any offset. The only compressible FP instructions with an offset
416 // are loads and stores, for which the offset applies to the GPR operand
417 // not the FPR operand.
418 assert(RegImm.Imm == 0);
419 unsigned Opcode = RISCV::FPR32RegClass.contains(RegImm.Reg)
420 ? RISCV::FSGNJ_S
421 : RISCV::FSGNJ_D;
422 BuildMI(MBB, MI, MI.getDebugLoc(), TII.get(Opcode), NewReg)
423 .addReg(RegImm.Reg)
424 .addReg(RegImm.Reg);
425 }
426
427 // Update the set of instructions to use the compressed register and
428 // compressible offset instead. These instructions should now be
429 // compressible.
430 // TODO: Update all uses if RegImm.Imm == 0? Not just those that are
431 // expected to become compressible.
432 for (MachineInstr *UpdateMI : MIs)
433 updateOperands(*UpdateMI, RegImm, NewReg);
434 }
435 }
436 return true;
437 }
438
439 /// Returns an instance of the Make Compressible Optimization pass.
createRISCVMakeCompressibleOptPass()440 FunctionPass *llvm::createRISCVMakeCompressibleOptPass() {
441 return new RISCVMakeCompressibleOpt();
442 }
443