Sorting Local Variables, Example | AMD 250 specification

25112 Rev. 3.06 September 2005

Software Optimization Guide for AMD64 Processors

2.19Sorting Local Variables

Optimization

Sort local variables according to their type sizes, declaring those with larger type sizes ahead of those with smaller type sizes.

Application

This optimization applies to:

•32-bit software

•64-bit software

Rationale

It can be helpful to presort local variables, if your compiler allocates local variables in the same order in which they are declared in the source code. If the first variable is allocated for natural alignment, all other variables are allocated contiguously in the order they are declared and are naturally aligned without padding.

Some compilers do not allocate variables in the order they are declared. In these cases, the compiler should automatically allocate variables that are naturally aligned with the minimum amount of padding. In addition, some compilers do not guarantee that the stack is aligned suitably for the largest type (that is, they do not guarantee quadword alignment), so that quadword operands might be misaligned, even if this technique is used and the compiler does allocate variables in the order they are declared.

Example

Avoid local variable declarations, when the variables are not declared in order of their type sizes:

short	ga, gu, gi;
long	foo, bar;
double	x,	y, z[3];
char	a,	b;
float	baz;

Instead, sort the declarations according to their type sizes (largest to smallest):

double	z[3];
double	x, y;
long	foo, bar;
float	baz;
short	ga, gu, gi;

Chapter 2

C and C++ Source-Level Optimizations

41

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AMD 250 manual Sorting Local Variables, Example

Contents

Software Optimization Guide For AMD64 Processors 2001 2005 Advanced Micro Devices, Inc. All rights reserved Contents General 64-Bit Optimizations Chapter Cache and Memory Optimizations Chapter Integer Optimizations 159 Chapter X87 Floating-Point Optimizations 237 Viii Appendix B Implementation of Write-Combining Index Software Optimization Guide for AMD64 Processors Tables Tables Xii Tables Figures Xiv Revision History Xvi Revision History Getting Started Quickly Intended Audience Using This Guide Numbering Systems Special InformationTypographic Notation Providing Feedback Important New Terms Primitive OperationsInternal Instruction Formats Types of Instructions Instructions, Macro-ops and Micro-opsMrom Guideline Key OptimizationsKey Optimizations by Rank Optimizations by Rank C++ Source-Level Optimizations C++ Source-Level Optimizations Optimization Declarations of Floating-Point ValuesApplication Rationale Using Arrays and Pointers Matrix Example Instead, use the equivalent array notation Additional Considerations Related Information Unrolling Small Loops Expression Order in Compound Branch Conditions Chapter C++ Source-Level Optimizations Long Logical Expressions in If Statements Arrange Boolean Operands for Quick Expression Evaluation If *p == y && strlenp Dynamic Memory Allocation Consideration Listing 3. Avoid Unnecessary Store-to-Load Dependencies Application Examples Matching Store and Load Size Listing 6. Preferred C++ Source-Level Optimizations Switch and Noncontiguous Case Expressions Example Related Information Arranging Cases by Probability of Occurrence Use of Function Prototypes Use of const Type Qualifier Rationale and Examples Generic Loop Hoisting Listing Chapter C++ Source-Level Optimizations Local Static Functions Explicit Parallelism in Code Listing 11. Preferred Extracting Common Subexpressions Listing 15. Example 2 Preferred Sorting and Padding C and C++ Structures Sorting and Padding C and C++ Structures C++ Source-Level Optimizations Sorting Local Variables Related Information Replacing Integer Division with Multiplication Listing 16. Avoid Frequently Dereferenced Pointer Arguments Listing 17. Preferred Array Indices Rational 23 32-Bit Integral Data Types Sign of Integer Operands Listing 20. Example 2 Avoid Accelerating Floating-Point Division and Square Root Examples Fast Floating-Point-to-Integer Conversion Listing 23. Slow Branches Dependent on Integer Comparisons Are Fast Speeding Up Branches Based on Comparisons Between Floats Comparisons against Positive Constant Comparisons among Two Floats Improving Performance in Linux Libraries Software Optimization Guide for AMD64 Processors General 64-Bit Optimizations 64-Bit Registers and Integer Arithmetic This code performs 64-bit addition using 32-bit registers ESP+8ESP+4 = multiplicand Background 64-Bit Arithmetic and Large-Integer Multiplication G1 = c3 E1 + f1 + g0 = c2 D1 + e0 + f0 = c1 D0 = c0 XMM END Text Segment 128-Bit Media Instructions and Floating-Point Operations 32-Bit Legacy GPRs and Small Unsigned Integers Chapter General 64-Bit Optimizations General 64-Bit Optimizations Instruction-Decoding Optimizations DirectPath Instructions Load-Execute Integer Instructions Optimization Load-Execute Instructions Movss xmm0, floatvar1 mulss xmm0, floatvar2 Application Branch Targets in Program Hot Spots 32/64-Bit vs -Bit Forms of the LEA Instruction Take Advantage of x86 and AMD64 Complex Addressing Modes Cmpb %al,0x68e35%r10,%r13 Short Instruction Encodings Partial-Register Reads and Writes Avoid Using Leave for Function Epilogues Functions That Allocate Local VariablesFunctions That Do not Allocate Local Variables Traditional function epilogue looks like this Alternatives to Shld Instruction Lea reg1, reg1*8+reg2 10 8-Bit Sign-Extended Immediate Values 11 8-Bit Sign-Extended Displacements NOP Code Padding with Operand-Size Override Instruction-Decoding Optimizations Cache and Memory Optimizations Examples-Store-to-Load-Forwarding Stalls Memory-Size MismatchesBit Avoid Avoid Preferred If the Contents of MM0 are No Longer Needed Examples-Large-to-small MismatchesPreferred If Stores Are Close to the Load Preferred If the Stores and Loads are Close Together, Option Natural Alignment of Data Objects Cache-Coherent Nonuniform Memory Access ccNUMA CPU0 OS Implications Dual-Core AMD Opteron Processor Configuration Multiprocessor Considerations Store-to-Load Forwarding Pitfalls-True Dependencies Store-to-Load Forwarding RestrictionsNarrow-to-Wide Store-Buffer Data-Forwarding Restriction 100 Wide-to-Narrow Store-Buffer Data-Forwarding Restriction Misaligned Store-Buffer Data-Forwarding Restriction101 One Supported Store-to-Load Forwarding Case High-Byte Store-Buffer Data-Forwarding RestrictionStore-to-Load Forwarding-False Dependencies 102 103 Summary of Store-to-Load-Forwarding Pitfalls to Avoid Prefetch Instructions Prefetching versus Preloading104 Hardware Prefetching Unit-Stride AccessPREFETCH/W versus PREFETCHNTA/T0/T1/T2 105 Prefetchw versus Prefetch Write-Combining Usage106 107 Multiple Prefetches 108 Determining Prefetch Distance Processor-Limited Code Memory-Limited Code Cache and Memory Optimizations Definitions 111 Prefetch at Least 64 Bytes Away from Surrounding Stores Streaming-Store/Non-Temporal Instructions 113 Write-combining How to Know If a Bank Conflict Exists L1 Data Cache Bank ConflictsFields Used to Address the Multibank L1 Data Cache 115 Placing Code and Data in the Same 64-Byte Cache Line 117 Sorting and Padding C and C++ Structures 118 119 Memory Copy Copying Small Data Structures120 121 Extend Arguments to 32 Bits Before Pushing onto Stack Stack ConsiderationsOptimized Stack Usage 122 123 Cache Issues when Writing Instruction Bytes to Memory 124 Interleave Loads and Stores 125 This Chapter 126 Density of Branches Align 127 128 Two-Byte Near-Return RET Instruction 129 Signed Integer ABS Function x = labsx Branches That Depend on Random DataUnsigned Integer min Function z = x y ? x y 130 131 Conditional Write 132 Pairing Call and Return 133 Recursive Functions 134 135 Nonzero Code-Segment Base Values Replacing Branches with Computation Muxing Constructs136 SSE Solution Preferred MMX Solution Avoid137 Example 1 C Code Sample Code Translated into AMD64 CodeExample 1 3DNow! Code Example 2 C Code Example 3 3DNow! Code Example 3 C CodeExample 4 C Code Example 4 3DNow! Code Example 5 C Code Example 5 3DNow! Code140 141 Loop Instruction 142 Far Control-Transfer Instructions 143 Chapter Scheduling Optimizations 144 Instruction Scheduling by Latency Loop Unrolling Loop Unrolling145 Example Complete Loop Unrolling Complete Loop UnrollingPartial Loop Unrolling Example Partial Loop Unrolling 147 Fadd 148 Deriving the Loop Control for Partially Unrolled Loops 149 Inline Functions 150 Additional Recommendations Address-Generation Interlocks Address-Generation Interlocks151 152 153 Movzx and Movsx 154 Pointer Arithmetic in Loops 155 156 157 Pushing Memory Data Directly onto the Stack 158 159 Chapter Integer Optimizations Multiplication by Reciprocal Division Utility Replacing Division with MultiplicationSigned Division Utility Unsigned Division Utility Algorithm Divisors 1 = d 231, Odd d Unsigned Division by Multiplication of ConstantAlgorithm Divisors 231 = d 161 Simpler Code for Restricted Dividend Signed Division by Multiplication of ConstantAlgorithm Divisors 2 = d Signed Division by Signed Division by -2n Signed Division by 2nRemainder of Signed Division by 2 or Remainder of Signed Division by 2n or -2n 164 Alternative Code for Multiplying by a Constant 165 166 Latency of Repeated String Instructions Repeated String InstructionsGuidelines for Repeated String Instructions 167 168 Use the Largest Possible Operand Size Using XOR to Clear Integer Registers Acceptable169 Bit Addition Efficient 64-Bit Integer Arithmetic in 32-Bit ModeBit Subtraction Bit Negation Bit Multiplication Bit Right ShiftBit Unsigned Division 171 172 173 Bit Signed Division 174 Bit Unsigned Remainder Computation 175 176 Bit Signed Remainder Computation 177 178 179 180 Integer Version Efficient Binary-to-ASCII Decimal Conversion Binary-to-ASCII Decimal Conversion Retaining Leading Zeros181 182 183 Binary-to-ASCII Decimal Conversion Suppressing Leading Zeros 184 185 186 Unsigned Integer Division 187 188 Signed Integer Division Example Code189 190 191 192 Optimizing Integer Division 193 Optimizing with Simd Instructions 194 195 Ensure All Packed Floating-Point Data are Aligned 196 Rationale-Single Precision 197 Rational-Double Precision 198 Use MOVLPx/MOVHPx Instructions for Unaligned Data Access 199 Use Movapd and Movaps Instead of Movupd and Movups Loop type Description 200 201 202 Double-Precision 32 ⋅ 32 Matrix Multiplication 203 204 205 206 207 208 Passing Data between MMX and 3DNow! Instructions 209 Storing Floating-Point Data in MMX Registers 210 Emms and Femms Usage XMM Text Segment 211 212 213 214 Single Precision Double Precision215 216 Clearing MMX and XMM Registers with XOR Instructions 217 218 219 220 221 Code below Puts the Floating Point Sign Mask 222 223 224 225 226 Pfpnacc 227 228 229 230 Listing 27 ⋅ 4 Matrix Multiplication SSE 231 XMM3 232 Listing 28 ⋅ 4 Matrix Multiplication 3DNow! Technology 233 234 235 236 237 X87 Floating-Point Optimizations 238 Using Multiplication Rather Than Division 239 Achieving Two Floating-Point Operations per Clock Cycle 240 241 242 Align and Pack DirectPath x87 Instructions 243 244 Floating-Point Compare Instructions 245 Using the Fxch Instruction Rather Than FST/FLD Pairs 246 Floating-Point Subexpression Elimination 247 Accumulating Precision-Sensitive Quantities in x87 Registers 248 Avoiding Extended-Precision Data 249 Key Microarchitecture Features Processor Block Diagram Superscalar Processor251 L1 Instruction Cache AMD Athlon 64 and AMD Opteron Processors Block Diagram L1 Instruction Cache Specifications by Processor Branch-Prediction Table253 Fetch-Decode Unit L1 Instruction TLB SpecificationsInstruction Control Unit Translation-Lookaside Buffer L1 Data TLB Specifications L2 TLB Specifications10 L1 Data Cache L1 Data Cache Specifications by Processor Integer SchedulerInteger Execution Unit 257 Floating-Point Scheduler Floating-Point Execution Unit Load-Store UnitFloating-Point Unit 259 16 L2 Cache Buses for AMD Athlon 64 and AMD Opteron Processor Integrated Memory ControllerHyperTransport Technology Interface 261 HyperTransport Technology Software Optimization Guide for AMD64 Processors 263 Write-Combining Definitions and Abbreviations Programming Details Write-combining Operations264 265 Write-Combining Completion Events 266 Sending Write-Buffer Data to the System 267 Optimizations 268 269 Appendix C Instruction Latencies Example Instruction Entry Understanding Instruction EntriesParts of the Instruction Entry 270 271 Interpreting Placeholders 272 Interpreting Latencies Integer Instructions Integer Instructions273 AAA ADD reg16/32/64, mem16/32/64 274 Bswap EAX/RAX/R8 275 276 277 Mem16/32/64 CMOVNP/CMOVPO reg16/32/64, reg16/32/64 278 CMP reg16/32/64, mreg16/32/64 279 280 281 282 283 JA/JNBE disp16/32 284 Lahf 285 286 Mfence 287 288 NOP Xchg EAX, EAX 289 290 291 292 Rdmsr 293Rdpmc Rdtsc Sahf 294 SBB reg16/32/64, mem16/32/64 295 296 297 298 STC 299STD STI Syscall 300Sysenter Sysexit 301 302 Xchg AX/EAX/RAX, SI/ESI/RSI/R14Xchg AX/EAX/RAX, DI/EDI/RDI/R15 MMX Technology Instructions MMX Technology Instructions303 Fmul 304 305 306 X87 Floating-Point Instructions X87 Floating-Point Instructions307 Fadd Fcompp 308Fadd Fcos Fdecstp 309 FincstpFADD/FMUL Fstore Finit 310 311 312 Fxch 313Fxtract FYL2X 3DNow! Technology Instructions 3DNow! Technology Instructions314 Femms 315 DNow! Technology Instructions 3DNow! Technology Extensions 3DNow! Technology Extensions316 SSE Instructions SSE Instructions317 318 319 320 321 Mem64 Prefetchnta mem8 0Fh 18h Mm-000-xxx DirectPath 322 Sfence 323 324 325 SSE2 Instructions SSE2 Instructions326 327 0FHFmul Fstore 328 Maskmovdqu 329 Fadd Fmul Fstore 330 331 332 Fadd Fmul 333 334 335 336 337 338 339 Fmul PunpckhbwFmul Punpckhdq 340 341 SSE3 Instructions SSE3 Instructions342 343 344 345 Fast-Write Optimizations 346 Fast-Write Optimizations for Graphics-Engine Programming 347 Cacheable-Memory Command Structure 348 349 Fast-Write Optimizations for Video-Memory Copies 350 351 Memory Optimizations 352 Northbridge Command Flow Optimizations for Texture-Map Copies to AGP Memory Optimizations for Vertex-Geometry Copies to AGP Memory353 354 Types of XMM-Register Data Types of SSE and SSE2 Instructions355 356 Half-Register Operations Zeroing Out an XMM Register Clearing XMM Registers357 358 359 Reuse of Dead Registers 360 Moving Data Between XMM Registers and GPRs 361 Saving and Restoring Registers of Unknown Format 362 SSE and SSE2 Copy Loops 363 Explicit Load Instructions Data Conversion Converting Scalar Values364 Converting Directly from Memory Converting Vector Values365 INT GPR FPS 366 367 Numerics 368 Index