循环抛出的编译时间 "parser stack overflow, program too complex"
Compile time for loop throwing "parser stack overflow, program too complex"
以下是编译时for循环：
namespace nstl
{
template <typename index_type, index_type begin, index_type stride, index_type... indices, typename function_type>
void compile_time_for_internal(std::index_sequence<indices...>, function_type function)
{
(function(begin + indices * stride), ...);
}
template <typename index_type, index_type begin, index_type end, index_type stride, typename function_type>
void compile_time_for(function_type function)
{
compile_time_for_internal<index_type, begin, stride>(std::make_index_sequence<end-begin>{}, function);
}
}
当我尝试 运行 这个最多包含 596 个元素的循环时，它起作用了：
int main(int argc, char** argv)
{
auto data = std::vector<double>(596);
std::iota(data.begin(), data.end(), 0);
auto sum(0.0);
nstl::compile_time_for<std::size_t, 0, 596, 1>([&] (const auto& i)
{
sum += data[i];
});
}
如果我使用 597 个元素，它会抛出“解析器堆栈溢出，程序太复杂”。我已经知道为什么会发生这种情况，例如折叠表达式使用某种递归在编译时扩展函数调用，但这只是理论。那么请问：这是什么原因呢？是否有避免此问题的解决方法？
Gcc 使用 597 编译您的代码也没有问题：https://godbolt.org/z/15ETd9rKM。我尝试加深，结果只看到godbolt kill掉了进程，gcc自己报错了。
Clang，报告以下内容error message：
<source>:11:40: fatal error: instantiating fold expression with 597 arguments exceeded expression nesting limit of 256
(function(begin + indices * stride), ...);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
<source>:16:3: note: in instantiation of function template specialization 'nstl::compile_time_for_internal<unsigned long, 0UL, 1UL, 0UL, 1UL, 2UL, 3UL, 4UL, 5UL, 6UL, 7UL, 8UL, 9UL, 10UL, 11UL, 12UL, 13UL, 14UL, 15UL, 16UL, 17UL, 18UL, 19UL, 20UL, 21UL, 22UL, 23UL, 24UL, 25UL, 26UL, 27UL, 28UL, 29UL, 30UL, 31UL, 32UL, 33UL, 34UL, 35UL, 36UL, 37UL, 38UL, 39UL, 40UL, 41UL, 42UL, 43UL, 44UL, 45UL, 46UL, 47UL, 48UL, 49UL, 50UL, 51UL, 52UL, 53UL, 54UL, 55UL, 56UL, 57UL, 58UL, 59UL, 60UL, 61UL, 62UL, 63UL, 64UL, 65UL, 66UL, 67UL, 68UL, 69UL, 70UL, 71UL, 72UL, 73UL, 74UL, 75UL, 76UL, 77UL, 78UL, 79UL, 80UL, 81UL, 82UL, 83UL, 84UL, 85UL, 86UL, 87UL, 88UL, 89UL, 90UL, 91UL, 92UL, 93UL, 94UL, 95UL, 96UL, 97UL, 98UL, 99UL, 100UL, 101UL, 102UL, 103UL, 104UL, 105UL, 106UL, 107UL, 108UL, 109UL, 110UL, 111UL, 112UL, 113UL, 114UL, 115UL, 116UL, 117UL, 118UL, 119UL, 120UL, 121UL, 122UL, 123UL, 124UL, 125UL, 126UL, 127UL, 128UL, 129UL, 130UL, 131UL, 132UL, 133UL, 134UL, 135UL, 136UL, 137UL, 138UL, 139UL, 140UL, 141UL, 142UL, 143UL, 144UL, 145UL, 146UL, 147UL, 148UL, 149UL, 150UL, 151UL, 152UL, 153UL, 154UL, 155UL, 156UL, 157UL, 158UL, 159UL, 160UL, 161UL, 162UL, 163UL, 164UL, 165UL, 166UL, 167UL, 168UL, 169UL, 170UL, 171UL, 172UL, 173UL, 174UL, 175UL, 176UL, 177UL, 178UL, 179UL, 180UL, 181UL, 182UL, 183UL, 184UL, 185UL, 186UL, 187UL, 188UL, 189UL, 190UL, 191UL, 192UL, 193UL, 194UL, 195UL, 196UL, 197UL, 198UL, 199UL, 200UL, 201UL, 202UL, 203UL, 204UL, 205UL, 206UL, 207UL, 208UL, 209UL, 210UL, 211UL, 212UL, 213UL, 214UL, 215UL, 216UL, 217UL, 218UL, 219UL, 220UL, 221UL, 222UL, 223UL, 224UL, 225UL, 226UL, 227UL, 228UL, 229UL, 230UL, 231UL, 232UL, 233UL, 234UL, 235UL, 236UL, 237UL, 238UL, 239UL, 240UL, 241UL, 242UL, 243UL, 244UL, 245UL, 246UL, 247UL, 248UL, 249UL, 250UL, 251UL, 252UL, 253UL, 254UL, 255UL, 256UL, 257UL, 258UL, 259UL, 260UL, 261UL, 262UL, 263UL, 264UL, 265UL, 266UL, 267UL, 268UL, 269UL, 270UL, 271UL, 272UL, 273UL, 274UL, 275UL, 276UL, 277UL, 278UL, 279UL, 280UL, 281UL, 282UL, 283UL, 284UL, 285UL, 286UL, 287UL, 288UL, 289UL, 290UL, 291UL, 292UL, 293UL, 294UL, 295UL, 296UL, 297UL, 298UL, 299UL, 300UL, 301UL, 302UL, 303UL, 304UL, 305UL, 306UL, 307UL, 308UL, 309UL, 310UL, 311UL, 312UL, 313UL, 314UL, 315UL, 316UL, 317UL, 318UL, 319UL, 320UL, 321UL, 322UL, 323UL, 324UL, 325UL, 326UL, 327UL, 328UL, 329UL, 330UL, 331UL, 332UL, 333UL, 334UL, 335UL, 336UL, 337UL, 338UL, 339UL, 340UL, 341UL, 342UL, 343UL, 344UL, 345UL, 346UL, 347UL, 348UL, 349UL, 350UL, 351UL, 352UL, 353UL, 354UL, 355UL, 356UL, 357UL, 358UL, 359UL, 360UL, 361UL, 362UL, 363UL, 364UL, 365UL, 366UL, 367UL, 368UL, 369UL, 370UL, 371UL, 372UL, 373UL, 374UL, 375UL, 376UL, 377UL, 378UL, 379UL, 380UL, 381UL, 382UL, 383UL, 384UL, 385UL, 386UL, 387UL, 388UL, 389UL, 390UL, 391UL, 392UL, 393UL, 394UL, 395UL, 396UL, 397UL, 398UL, 399UL, 400UL, 401UL, 402UL, 403UL, 404UL, 405UL, 406UL, 407UL, 408UL, 409UL, 410UL, 411UL, 412UL, 413UL, 414UL, 415UL, 416UL, 417UL, 418UL, 419UL, 420UL, 421UL, 422UL, 423UL, 424UL, 425UL, 426UL, 427UL, 428UL, 429UL, 430UL, 431UL, 432UL, 433UL, 434UL, 435UL, 436UL, 437UL, 438UL, 439UL, 440UL, 441UL, 442UL, 443UL, 444UL, 445UL, 446UL, 447UL, 448UL, 449UL, 450UL, 451UL, 452UL, 453UL, 454UL, 455UL, 456UL, 457UL, 458UL, 459UL, 460UL, 461UL, 462UL, 463UL, 464UL, 465UL, 466UL, 467UL, 468UL, 469UL, 470UL, 471UL, 472UL, 473UL, 474UL, 475UL, 476UL, 477UL, 478UL, 479UL, 480UL, 481UL, 482UL, 483UL, 484UL, 485UL, 486UL, 487UL, 488UL, 489UL, 490UL, 491UL, 492UL, 493UL, 494UL, 495UL, 496UL, 497UL, 498UL, 499UL, 500UL, 501UL, 502UL, 503UL, 504UL, 505UL, 506UL, 507UL, 508UL, 509UL, 510UL, 511UL, 512UL, 513UL, 514UL, 515UL, 516UL, 517UL, 518UL, 519UL, 520UL, 521UL, 522UL, 523UL, 524UL, 525UL, 526UL, 527UL, 528UL, 529UL, 530UL, 531UL, 532UL, 533UL, 534UL, 535UL, 536UL, 537UL, 538UL, 539UL, 540UL, 541UL, 542UL, 543UL, 544UL, 545UL, 546UL, 547UL, 548UL, 549UL, 550UL, 551UL, 552UL, 553UL, 554UL, 555UL, 556UL, 557UL, 558UL, 559UL, 560UL, 561UL, 562UL, 563UL, 564UL, 565UL, 566UL, 567UL, 568UL, 569UL, 570UL, 571UL, 572UL, 573UL, 574UL, 575UL, 576UL, 577UL, 578UL, 579UL, 580UL, 581UL, 582UL, 583UL, 584UL, 585UL, 586UL, 587UL, 588UL, 589UL, 590UL, 591UL, 592UL, 593UL, 594UL, 595UL, 596UL, (lambda at <source>:27:50)>' requested here
compile_time_for_internal<index_type, begin, stride>(std::make_index_sequence<end-begin>{}, function);
^
<source>:27:9: note: in instantiation of function template specialization 'nstl::compile_time_for<unsigned long, 0UL, 597UL, 1UL, (lambda at <source>:27:50)>' requested here
nstl::compile_time_for<std::size_t, 0, 597, 1>([&] (const auto& i)
^
<source>:11:40: note: use -fbracket-depth=N to increase maximum nesting level
(function(begin + indices * stride), ...);
^
1 error generated.
Compiler returned: 1
确实，当您使用 -fbracket-depth=600 时，clang 也会编译代码：https://godbolt.org/z/59Y8ze6a8.
我没有尝试过 MSVC，但我想所有主要的 3 都有一个标志，可以让你改变那个限制。
更新： 我对 MSCV 不是很熟悉（它是唯一一个在我尝试时报告“程序太复杂”的消息，所以我想它是您正在使用的那个）。我一直在寻找一个标志来增加限制，但我没有找到。所以我假设所有主要 3 都有这样的标志似乎是错误的。需要解决方法。
您可以手动展开循环以减少模板的深度：
#include <iostream>
#include <utility>
#include <vector>
#include <numeric>
namespace nstl
{
template <typename index_type, index_type begin, index_type stride, index_type... indices, typename function_type>
void compile_time_for_internal(std::index_sequence<indices...>, function_type function)
{
(function(begin + indices*2*stride),...);
(function(begin + (indices*2+1)*stride),...);
}
template <typename index_type, index_type begin, index_type end, index_type stride, typename function_type>
void compile_time_for(function_type function)
{
compile_time_for_internal<index_type, begin, stride>(std::make_index_sequence<(end-begin)/2>{}, function);
}
}
int main(int argc, char** argv)
{
auto data = std::vector<double>(600);
std::iota(data.begin(), data.end(), 0);
auto sum(0.0);
nstl::compile_time_for<std::size_t, 0, 600, 1>([&] (const auto& i)
{
sum += data[i];
});
std::cout << sum << "\n";
sum = 0.0;
for (int i=0; i < 100; ++i) sum+= i;
std::cout << sum;
}
这仅适用于偶数次迭代。为了也允许奇数次迭代，您需要区分这两种情况并单独处理最后一次迭代。当然，您可以展开更多迭代以进一步突破极限。
Live Demo with clang, this time only -fbracket-depth=300 (note: 600 iterations)。不幸的是，由于一些内部编译器资源管理器错误，我无法使用 MSVC 对其进行测试。不过，请注意 clang 需要 -fbracket-depth=600 用于未展开的版本，而它可以处理深度为一半的展开版本。
以下是编译时for循环：
namespace nstl
{
template <typename index_type, index_type begin, index_type stride, index_type... indices, typename function_type>
void compile_time_for_internal(std::index_sequence<indices...>, function_type function)
{
(function(begin + indices * stride), ...);
}
template <typename index_type, index_type begin, index_type end, index_type stride, typename function_type>
void compile_time_for(function_type function)
{
compile_time_for_internal<index_type, begin, stride>(std::make_index_sequence<end-begin>{}, function);
}
}
当我尝试 运行 这个最多包含 596 个元素的循环时，它起作用了：
int main(int argc, char** argv)
{
auto data = std::vector<double>(596);
std::iota(data.begin(), data.end(), 0);
auto sum(0.0);
nstl::compile_time_for<std::size_t, 0, 596, 1>([&] (const auto& i)
{
sum += data[i];
});
}
如果我使用 597 个元素，它会抛出“解析器堆栈溢出，程序太复杂”。我已经知道为什么会发生这种情况，例如折叠表达式使用某种递归在编译时扩展函数调用，但这只是理论。那么请问：这是什么原因呢？是否有避免此问题的解决方法？
Gcc 使用 597 编译您的代码也没有问题：https://godbolt.org/z/15ETd9rKM。我尝试加深，结果只看到godbolt kill掉了进程，gcc自己报错了。
Clang，报告以下内容error message：
<source>:11:40: fatal error: instantiating fold expression with 597 arguments exceeded expression nesting limit of 256
(function(begin + indices * stride), ...);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
<source>:16:3: note: in instantiation of function template specialization 'nstl::compile_time_for_internal<unsigned long, 0UL, 1UL, 0UL, 1UL, 2UL, 3UL, 4UL, 5UL, 6UL, 7UL, 8UL, 9UL, 10UL, 11UL, 12UL, 13UL, 14UL, 15UL, 16UL, 17UL, 18UL, 19UL, 20UL, 21UL, 22UL, 23UL, 24UL, 25UL, 26UL, 27UL, 28UL, 29UL, 30UL, 31UL, 32UL, 33UL, 34UL, 35UL, 36UL, 37UL, 38UL, 39UL, 40UL, 41UL, 42UL, 43UL, 44UL, 45UL, 46UL, 47UL, 48UL, 49UL, 50UL, 51UL, 52UL, 53UL, 54UL, 55UL, 56UL, 57UL, 58UL, 59UL, 60UL, 61UL, 62UL, 63UL, 64UL, 65UL, 66UL, 67UL, 68UL, 69UL, 70UL, 71UL, 72UL, 73UL, 74UL, 75UL, 76UL, 77UL, 78UL, 79UL, 80UL, 81UL, 82UL, 83UL, 84UL, 85UL, 86UL, 87UL, 88UL, 89UL, 90UL, 91UL, 92UL, 93UL, 94UL, 95UL, 96UL, 97UL, 98UL, 99UL, 100UL, 101UL, 102UL, 103UL, 104UL, 105UL, 106UL, 107UL, 108UL, 109UL, 110UL, 111UL, 112UL, 113UL, 114UL, 115UL, 116UL, 117UL, 118UL, 119UL, 120UL, 121UL, 122UL, 123UL, 124UL, 125UL, 126UL, 127UL, 128UL, 129UL, 130UL, 131UL, 132UL, 133UL, 134UL, 135UL, 136UL, 137UL, 138UL, 139UL, 140UL, 141UL, 142UL, 143UL, 144UL, 145UL, 146UL, 147UL, 148UL, 149UL, 150UL, 151UL, 152UL, 153UL, 154UL, 155UL, 156UL, 157UL, 158UL, 159UL, 160UL, 161UL, 162UL, 163UL, 164UL, 165UL, 166UL, 167UL, 168UL, 169UL, 170UL, 171UL, 172UL, 173UL, 174UL, 175UL, 176UL, 177UL, 178UL, 179UL, 180UL, 181UL, 182UL, 183UL, 184UL, 185UL, 186UL, 187UL, 188UL, 189UL, 190UL, 191UL, 192UL, 193UL, 194UL, 195UL, 196UL, 197UL, 198UL, 199UL, 200UL, 201UL, 202UL, 203UL, 204UL, 205UL, 206UL, 207UL, 208UL, 209UL, 210UL, 211UL, 212UL, 213UL, 214UL, 215UL, 216UL, 217UL, 218UL, 219UL, 220UL, 221UL, 222UL, 223UL, 224UL, 225UL, 226UL, 227UL, 228UL, 229UL, 230UL, 231UL, 232UL, 233UL, 234UL, 235UL, 236UL, 237UL, 238UL, 239UL, 240UL, 241UL, 242UL, 243UL, 244UL, 245UL, 246UL, 247UL, 248UL, 249UL, 250UL, 251UL, 252UL, 253UL, 254UL, 255UL, 256UL, 257UL, 258UL, 259UL, 260UL, 261UL, 262UL, 263UL, 264UL, 265UL, 266UL, 267UL, 268UL, 269UL, 270UL, 271UL, 272UL, 273UL, 274UL, 275UL, 276UL, 277UL, 278UL, 279UL, 280UL, 281UL, 282UL, 283UL, 284UL, 285UL, 286UL, 287UL, 288UL, 289UL, 290UL, 291UL, 292UL, 293UL, 294UL, 295UL, 296UL, 297UL, 298UL, 299UL, 300UL, 301UL, 302UL, 303UL, 304UL, 305UL, 306UL, 307UL, 308UL, 309UL, 310UL, 311UL, 312UL, 313UL, 314UL, 315UL, 316UL, 317UL, 318UL, 319UL, 320UL, 321UL, 322UL, 323UL, 324UL, 325UL, 326UL, 327UL, 328UL, 329UL, 330UL, 331UL, 332UL, 333UL, 334UL, 335UL, 336UL, 337UL, 338UL, 339UL, 340UL, 341UL, 342UL, 343UL, 344UL, 345UL, 346UL, 347UL, 348UL, 349UL, 350UL, 351UL, 352UL, 353UL, 354UL, 355UL, 356UL, 357UL, 358UL, 359UL, 360UL, 361UL, 362UL, 363UL, 364UL, 365UL, 366UL, 367UL, 368UL, 369UL, 370UL, 371UL, 372UL, 373UL, 374UL, 375UL, 376UL, 377UL, 378UL, 379UL, 380UL, 381UL, 382UL, 383UL, 384UL, 385UL, 386UL, 387UL, 388UL, 389UL, 390UL, 391UL, 392UL, 393UL, 394UL, 395UL, 396UL, 397UL, 398UL, 399UL, 400UL, 401UL, 402UL, 403UL, 404UL, 405UL, 406UL, 407UL, 408UL, 409UL, 410UL, 411UL, 412UL, 413UL, 414UL, 415UL, 416UL, 417UL, 418UL, 419UL, 420UL, 421UL, 422UL, 423UL, 424UL, 425UL, 426UL, 427UL, 428UL, 429UL, 430UL, 431UL, 432UL, 433UL, 434UL, 435UL, 436UL, 437UL, 438UL, 439UL, 440UL, 441UL, 442UL, 443UL, 444UL, 445UL, 446UL, 447UL, 448UL, 449UL, 450UL, 451UL, 452UL, 453UL, 454UL, 455UL, 456UL, 457UL, 458UL, 459UL, 460UL, 461UL, 462UL, 463UL, 464UL, 465UL, 466UL, 467UL, 468UL, 469UL, 470UL, 471UL, 472UL, 473UL, 474UL, 475UL, 476UL, 477UL, 478UL, 479UL, 480UL, 481UL, 482UL, 483UL, 484UL, 485UL, 486UL, 487UL, 488UL, 489UL, 490UL, 491UL, 492UL, 493UL, 494UL, 495UL, 496UL, 497UL, 498UL, 499UL, 500UL, 501UL, 502UL, 503UL, 504UL, 505UL, 506UL, 507UL, 508UL, 509UL, 510UL, 511UL, 512UL, 513UL, 514UL, 515UL, 516UL, 517UL, 518UL, 519UL, 520UL, 521UL, 522UL, 523UL, 524UL, 525UL, 526UL, 527UL, 528UL, 529UL, 530UL, 531UL, 532UL, 533UL, 534UL, 535UL, 536UL, 537UL, 538UL, 539UL, 540UL, 541UL, 542UL, 543UL, 544UL, 545UL, 546UL, 547UL, 548UL, 549UL, 550UL, 551UL, 552UL, 553UL, 554UL, 555UL, 556UL, 557UL, 558UL, 559UL, 560UL, 561UL, 562UL, 563UL, 564UL, 565UL, 566UL, 567UL, 568UL, 569UL, 570UL, 571UL, 572UL, 573UL, 574UL, 575UL, 576UL, 577UL, 578UL, 579UL, 580UL, 581UL, 582UL, 583UL, 584UL, 585UL, 586UL, 587UL, 588UL, 589UL, 590UL, 591UL, 592UL, 593UL, 594UL, 595UL, 596UL, (lambda at <source>:27:50)>' requested here
compile_time_for_internal<index_type, begin, stride>(std::make_index_sequence<end-begin>{}, function);
^
<source>:27:9: note: in instantiation of function template specialization 'nstl::compile_time_for<unsigned long, 0UL, 597UL, 1UL, (lambda at <source>:27:50)>' requested here
nstl::compile_time_for<std::size_t, 0, 597, 1>([&] (const auto& i)
^
<source>:11:40: note: use -fbracket-depth=N to increase maximum nesting level
(function(begin + indices * stride), ...);
^
1 error generated.
Compiler returned: 1
确实，当您使用 -fbracket-depth=600 时，clang 也会编译代码：https://godbolt.org/z/59Y8ze6a8.
我没有尝试过 MSVC，但我想所有主要的 3 都有一个标志，可以让你改变那个限制。
更新： 我对 MSCV 不是很熟悉（它是唯一一个在我尝试时报告“程序太复杂”的消息，所以我想它是您正在使用的那个）。我一直在寻找一个标志来增加限制，但我没有找到。所以我假设所有主要 3 都有这样的标志似乎是错误的。需要解决方法。
您可以手动展开循环以减少模板的深度：
#include <iostream>
#include <utility>
#include <vector>
#include <numeric>
namespace nstl
{
template <typename index_type, index_type begin, index_type stride, index_type... indices, typename function_type>
void compile_time_for_internal(std::index_sequence<indices...>, function_type function)
{
(function(begin + indices*2*stride),...);
(function(begin + (indices*2+1)*stride),...);
}
template <typename index_type, index_type begin, index_type end, index_type stride, typename function_type>
void compile_time_for(function_type function)
{
compile_time_for_internal<index_type, begin, stride>(std::make_index_sequence<(end-begin)/2>{}, function);
}
}
int main(int argc, char** argv)
{
auto data = std::vector<double>(600);
std::iota(data.begin(), data.end(), 0);
auto sum(0.0);
nstl::compile_time_for<std::size_t, 0, 600, 1>([&] (const auto& i)
{
sum += data[i];
});
std::cout << sum << "\n";
sum = 0.0;
for (int i=0; i < 100; ++i) sum+= i;
std::cout << sum;
}
这仅适用于偶数次迭代。为了也允许奇数次迭代，您需要区分这两种情况并单独处理最后一次迭代。当然，您可以展开更多迭代以进一步突破极限。
Live Demo with clang, this time only -fbracket-depth=300 (note: 600 iterations)。不幸的是，由于一些内部编译器资源管理器错误，我无法使用 MSVC 对其进行测试。不过，请注意 clang 需要 -fbracket-depth=600 用于未展开的版本，而它可以处理深度为一半的展开版本。