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General Optimization Guidelines 2
2-15
Branch Prediction
Branch optimizations have a significant impact on performance. By
understanding the flow of branches and improving the predictability of
branches, you can increase the speed of code significantly.
Optimizations that help branch prediction are:
• Keep code and data on separate pages (a very important item, see
more details in the “Memory Accesses” section).
• Whenever possible, eliminate branches.
• Arrange code to be consistent with the static branch prediction
algorithm.
• Use the pause instruction in spin-wait loops.
• Inline functions and pair up calls and returns.
• Unroll as necessary so that repeatedly-executed loops have sixteen
or fewer iterations, unless this causes an excessive code size
increase.
• Separate branches so that they occur no more frequently than every
three
μops where possible.
Eliminating Branches
Eliminating branches improves performance because it:
• reduces the possibility of mispredictions
• reduces the number of required branch target buffer (BTB) entries;
conditional branches, which are never taken, do not consume BTB
resources
There are four principal ways of eliminating branches:
• arrange code to make basic blocks contiguous
• unroll loops, as discussed in the “Loop Unrolling” section
• use the cmov instruction
• use the setcc instruction
- IA-32 Intel® Architecture 1
- Optimization Reference 1
- Contents 3
- Appendix DStack Alignment 14
- Examples 15
- Introduction 23
- Tuning Your Application 24
- About This Manual 24
- Related Documentation 27
- Notational Conventions 28
- IA-32 Intel 29
- Architecture 29
- Processor Family Overview 29
- SIMD Technology 30
- Y4 Y3 Y2 Y1 31
- OP OP OP OP 31
- • inherently parallel 32
- Summary of SIMD Technologies 33
- Streaming SIMD Extensions 2 34
- Streaming SIMD Extensions 3 34
- Intel NetBurst 36
- Microarchitecture 36
- )UHTXHQWO\XVHGSDWKV 38
- /HVVIUHTXHQWO\XVHGSDWKV 38
- The Front End 39
- The Out-of-order Core 40
- Retirement 40
- Front End Pipeline Detail 41
- Execution Trace Cache 42
- Branch Prediction 43
- Execution Core Detail 44
- Data Prefetch 49
- • multiple outstanding misses 52
- • buffering of writes 52
- Pentium 54
- • fetch/decode unit 55
- • instruction cache 55
- Data Prefetching 57
- Out-of-Order Core 58
- Microarchitecture of Intel 59
- Core™ Solo and 59
- Core™ Duo Processors 59
- • Power-optimized bus 60
- • Data Prefetch 60
- • Micro-op fusion 60
- • operational fairness 64
- Shared Resources 65
- Front End Pipeline 66
- Multi-Core Processors 67
- Load and Store Operations 70
- General Optimization 73
- Optimize Memory Access 77
- Enable Vectorization 79
- Performance Tools 81
- VTune™ Performance Analyzer 82
- Processor Perspectives 83
- A and B. If the condition is 88
- Spin-Wait and Idle Loops 90
- Static Prediction 91
- Inlining, Calls and Returns 94
- Branch Type Selection 95
- Loop Unrolling 98
- • inlining where appropriate 100
- Memory Accesses 101
- Line 029e7100h 103
- Line 029e70c0h 103
- Line 029e7140h 103
- Store Forwarding 104
- Alignment 105
- Figure 2-2 106
- Example 2-14 108
- • parameter passing 110
- Data Layout Optimizations 111
- Stack Alignment 114
- Aliasing Cases in the Pentium 117
- 4 and Intel 117
- Processors 117
- Mixing Code and Data 119
- Write Combining 120
- Locality Enhancement 122
- Minimizing Bus Latency 124
- • software prefetch for data 127
- Cacheability Instructions 128
- Applications 129
- • arithmetic overflow 132
- • arithmetic underflow 132
- • denormalized operand 132
- Floating-point Modes 134
- Core Duo Processors 142
- Memory Operands 143
- Floating-Point Stalls 144
- Instruction Selection 145
- Complex Instructions 146
- Use of the lea Instruction 146
- Flag Register Accesses 147
- Integer Divide 148
- Alternate Sequence without 150
- Partial Register Stall 150
- • Operand size prefix (0x66) 152
- • Address size prefix (0x67) 152
- REP Prefix and Data Movement 153
- • Throughput per iteration: 154
- • Address alignment: 154
- • Cache eviction: 155
- Destination 157
- • immediate constant 158
- • base register 158
- • scaled index register 158
- Clearing Registers 159
- Compares 159
- Floating Point/SIMD Operands 160
- Prolog Sequences 162
- Instruction Scheduling 163
- Spill Scheduling 164
- Vectorization 165
- • avoid global pointers 166
- • avoid global variables 166
- Miscellaneous 167
- User/Source Coding Rules 169
- PUSH, CALL, RET). 2-84 179
- Tuning Suggestions 180
- Coding for SIMD 181
- Architectures 181
- Technologies 182
- bool OSSupportCheck() { 184
- Programming 188
- Identifying Hot Spots 190
- Coding Techniques 192
- Coding Methodologies 193
- Assembly 195
- Intrinsics 195
- +”, “>>”) 197
- Automatic Vectorization 198
- Stack and Data Alignment 200
- __m128* datatypes 202
- __m128* 203
- Compiler-Supported Alignment 204
- Improving Memory Utilization 207
- SoA Data Structure 208
- Strip Mining 212
- Example 3-19 Strip Mined Code 213
- Loop Blocking 214
- Example 3-20 Loop Blocking 215
- A. Original Loop 215
- Blocking 216
- Tuning the Final Application 219
- Optimizing for SIMD Integer 221
- Using the EMMS Instruction 223
- Data Alignment 226
- Signed Unpack 227
- MM/M64 mm 229
- Non-Interleaved Unpack 231
- Extract Word 233
- Insert Word 234
- Figure 4-6 pinsrw Instruction 235
- Move Byte Mask to Integer 236
- 55 47 39 23 15 7 237
- X4 X3 X2 X1 238
- X1 X2 X3 X4 238
- Generating Constants 241
- Building Blocks 243
- Absolute Value 245
- 0x8000800080008000 246
- Highly Efficient Clipping 247
- Signed Word 249
- Packed Multiply High Unsigned 250
- Packed Average (Byte/Word) 251
- Packed 32*32 Multiply 253
- Packed 64-bit Add/Subtract 253
- 128-bit Shifts 253
- Memory Optimizations 254
- Partial Memory Accesses 255
- Instruction 259
- Optimizing for SIMD 263
- Floating-point Applications 263
- Planning Considerations 264
- Scalar Floating-point Code 265
- Data Swizzling 271
- Example 5-3 Swizzling Data 272
- Data Deswizzling 276
- Instructions 277
- Instructions (continued) 278
- Functions 279
- Horizontal ADD Using SSE 280
- C1 C2 C3 C4 D1 D2 D3 D4 281
- C1 C2 D1 D2 C3 C4 D3 D4 281
- MXCSR register should be 283
- SSE3 and Complex Arithmetics 285
- Optimizing Cache Usage 291
- Optimizing Cache Usage 6 293
- Hardware Prefetching of Data 294
- Prefetch 296
- Implementation 298
- Cacheability Control 299
- Fencing 300
- Streaming Non-temporal Stores 300
- WB) or Write-Through (WT) 301
- WC semantics) 301
- Write-Combining 302
- Streaming Store Usage Models 303
- • hand-crafted code 305
- The lfence Instruction 306
- The mfence Instruction 306
- The clflush Instruction 307
- Software-controlled Prefetch 308
- Hardware Prefetch 309
- Constant Stride 311
- Non-Adjacent Passes Loops 326
- 60 invis 332
- • write-once (non-temporal) 333
- Cache Management 334
- Video Encoder 335
- Video Decoder 335
- • alignment of data 337
- • cache size 337
- Bit Location Name Meaning 344
- • Determine prefetch stride 345
- Parameters 346
- Multi-Core and 347
- Hyper-Threading Technology 347
- Performance and Usage Models 348
- Single Thread 349
- Multi-Thread on MP 349
- Multitasking Environment 350
- • workload 352
- • thread interaction 352
- • hardware utilization 352
- • domain decomposition 353
- • functional decomposition 353
- Functional Decomposition 354
- P(1)P(1) C(1)C(1)P(1) 355
- P: producer 356
- C: consumer 356
- Thread 0 358
- Thread 1 358
- Optimization Guidelines 362
- Thread Synchronization 365
- Optimization with Spin-Locks 371
- PAUSE instruction in the 372
- Example 7-5 373
- System Bus Optimization 379
- Conserve Bus Bandwidth 380
- Memory Optimization 384
- Shared-Memory Optimization 385
- 4 KB in each thread 388
- Per-thread Stack Offset 390
- Per-instance Stack Offset 392
- Front-end Optimization 394
- Resources 395
- Processor 397
- Processor (Contd.) 398
- Sharing the Same Cache 401
- 64-bit Mode Coding 409
- Guidelines 409
- Only When Necessary 410
- Assembly/Compiler Coding rule 411
- 64-Bit Arithmetic 412
- Assembly/Compiler Coding Rule 413
- Possible 414
- Using Software Prefetch 414
- Power Optimization for 415
- Mobile Usages 415
- Mobile Usage Scenarios 416
- ACPI C-States 418
- Reducing Amount of Work 423
- • Switch off unused devices 424
- Technology 426
- Enabling Intel 428
- Enhanced Deeper Sleep 428
- Multi-Core Considerations 429
- (C1-C4) 433
- Application Performance 435
- Compilers 436
- Code Optimization Options 437
- Vectorizer Switch Options 439
- Multithreading with OpenMP* 440
- VTune™ Performance Analyzer 442
- Sampling 443
- Event-based Sampling 444
- Workload Characterization 445
- Call Graph 447
- Performance Libraries 448
- Benefits Summary 449
- Optimizations with the Intel 450
- Enhanced Debugger (EDB) 451
- Threading Tools 451
- Thread Profiler 453
- Software College 454
- Using Performance Monitoring 455
- Bogus, Non-bogus, Retire 456
- Bus Ratio 456
- Counting Clocks 458
- Non-Halted Clockticks 459
- Non-Sleep Clockticks 460
- Time Stamp Counter 461
- Microarchitecture Notes 462
- Side Bus 464
- Reads due to program loads 465
- Writebacks (dirty evictions) 466
- Usage Notes on Bus Activities 469
- Tags for replay_event 500
- Tags for front_end_event 502
- Tags for execution_event 502
- Technology 504
- Parallel Counting 505
- Parallel Counting (continued) 506
- Intel Core Duo processors 510
- Ratio Interpretation 511
- Notes on Selected Events 512
- Throughput 515
- Overview 516
- PADDQ and PMULUDQ, each have 517
- Definitions 518
- Latency and Throughput 518
- See “Table Footnotes” 520
- Instructions (continued) 524
- Table Footnotes 533
- Stack Alignment D 539
- & 0x0f) == 0x08 542
- Stack Frame Optimizations 545
- Inlined Assembly and ebx 546
- Mathematics of Prefetch 547
- Scheduling Distance 547
- Mathematical Model for PSD 548
- L2 lookup miss latency 550
- • Optimize T 551
- No Preloading or Prefetch 552
- Front-Side Bus 553
- Execution pipeline 553
- Execution cycles 553
- Compute Bound (Case: T 554
- >= T 556
- INTEL SALES OFFICES 567
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