> but the optimized code only appears to be checking the stack limit at > the function entry, not in the loop itself.
Yes. As I said in my previous email: optimizing compiler eliminates stack checks that are dominated by calls because called functions should at least have a stack check in the prologue. However there seems to be a minor issue here. Apparently HandleApiCall builtin does not perform any stack checks thus breaking HStackCheckEliminator assumption that every call implies a stack check. If print in your example is a JS function everything is fine because JS function has a stack check in the prologue. But if print is an API function then this loop will have no interruption point which is bad. -- Vyacheslav Egorov On Wed, Aug 31, 2011 at 10:39 PM, Kyle Morgan <[email protected]> wrote: > Hi Vyacheslav, > > Allow me to demonstrate what I mean. I ran the v8 shell with code > containing the following function. > > function loop() { > for(var i = 0; i < 5; ++i) { > print(i); > } > } > > It appears to be emitting the following code (optimized and unoptimized). > > --- Code --- > kind = FUNCTION > name = loop > Instructions (size = 196) > 0x7f6f3e26fca0 0 55 push rbp > 0x7f6f3e26fca1 1 4889e5 REX.W movq rbp,rsp > 0x7f6f3e26fca4 4 56 push rsi > 0x7f6f3e26fca5 5 57 push rdi > 0x7f6f3e26fca6 6 41ff7598 push [r13-0x68] > 0x7f6f3e26fcaa 10 493b6508 REX.W cmpq rsp,[r13+0x8] > 0x7f6f3e26fcae 14 7305 jnc 21 (0x7f6f3e26fcb5) > 0x7f6f3e26fcb0 16 e82bf4fdff call 0x7f6f3e24f0e0 ;; debug: > statement 36 > ;; code: STUB, > StackCheckStub, minor: 0 > 0x7f6f3e26fcb5 21 33c0 xorl rax,rax > 0x7f6f3e26fcb7 23 488945e8 REX.W movq [rbp-0x18],rax > 0x7f6f3e26fcbb 27 e94d000000 jmp 109 (0x7f6f3e26fd0d) > 0x7f6f3e26fcc0 32 ff7627 push [rsi+0x27] > 0x7f6f3e26fcc3 35 ff75e8 push [rbp-0x18] > 0x7f6f3e26fcc6 38 48b9919790636f7f0000 REX.W movq rcx,0x7f6f63909791 > ;; object: 0x7f6f63909791 <String[5]: print> > 0x7f6f3e26fcd0 48 e80b98ffff call 0x7f6f3e2694e0 ;; debug: > statement 76 > ;; code: > contextual, CALL_IC, UNINITIALIZED, in_loop, argc = 1 > 0x7f6f3e26fcd5 53 488b75f8 REX.W movq rsi,[rbp-0x8] > 0x7f6f3e26fcd9 57 488b45e8 REX.W movq rax,[rbp-0x18] > 0x7f6f3e26fcdd 61 a801 test al,0x1 > 0x7f6f3e26fcdf 63 7405 jz 70 (0x7f6f3e26fce6) > 0x7f6f3e26fce1 65 e89a83feff call 0x7f6f3e258080 ;; debug: > statement 43 > ;; debug: > position 67 > ;; code: STUB, > ToNumberStub, minor: 0 > 0x7f6f3e26fce6 70 4c01e0 REX.W addq rax,r12 > 0x7f6f3e26fce9 73 7004 jo 79 (0x7f6f3e26fcef) > 0x7f6f3e26fceb 75 a801 test al,0x1 > 0x7f6f3e26fced 77 720d jc 92 (0x7f6f3e26fcfc) > 0x7f6f3e26fcef 79 4c29e0 REX.W subq rax,r12 > 0x7f6f3e26fcf2 82 4c89e2 REX.W movq rdx,r12 > 0x7f6f3e26fcf5 85 e80658feff call 0x7f6f3e255500 ;; code: > BINARY_OP_IC, UNINITIALIZED (id = 30) > 0x7f6f3e26fcfa 90 a80d test al,0xd > 0x7f6f3e26fcfc 92 488945e8 REX.W movq [rbp-0x18],rax > 0x7f6f3e26fd00 96 493b6508 REX.W cmpq rsp,[r13+0x8] > 0x7f6f3e26fd04 100 7307 jnc 109 (0x7f6f3e26fd0d) > 0x7f6f3e26fd06 102 e8d5f3fdff call 0x7f6f3e24f0e0 ;; code: STUB, > StackCheckStub, minor: 0 > 0x7f6f3e26fd0b 107 a801 test al,0x1 > 0x7f6f3e26fd0d 109 ff75e8 push [rbp-0x18] > 0x7f6f3e26fd10 112 4b8d04a4 REX.W leaq rax,[r12+r12*4] > 0x7f6f3e26fd14 116 5a pop rdx > 0x7f6f3e26fd15 117 488bca REX.W movq rcx,rdx > 0x7f6f3e26fd18 120 480bc8 REX.W orq rcx,rax > 0x7f6f3e26fd1b 123 f6c101 testb rcx,0x1 > 0x7f6f3e26fd1e 126 730a jnc 138 (0x7f6f3e26fd2a) > 0x7f6f3e26fd20 128 483bd0 REX.W cmpq rdx,rax > 0x7f6f3e26fd23 131 7c9b jl 32 (0x7f6f3e26fcc0) > 0x7f6f3e26fd25 133 e91d000000 jmp 167 (0x7f6f3e26fd47) > 0x7f6f3e26fd2a 138 e8313cfeff call 0x7f6f3e253960 ;; debug: > position 60 > ;; code: > COMPARE_IC, UNINITIALIZED (id = 23) > 0x7f6f3e26fd2f 143 a811 test al,0x11 > 0x7f6f3e26fd31 145 eb0b jmp 158 (0x7f6f3e26fd3e) > 0x7f6f3e26fd33 147 493b45b0 REX.W cmpq rax,[r13-0x50] > 0x7f6f3e26fd37 151 7487 jz 32 (0x7f6f3e26fcc0) > 0x7f6f3e26fd39 153 e909000000 jmp 167 (0x7f6f3e26fd47) > 0x7f6f3e26fd3e 158 4885c0 REX.W testq rax,rax > 0x7f6f3e26fd41 161 0f8c79ffffff jl 32 (0x7f6f3e26fcc0) > 0x7f6f3e26fd47 167 498b4598 REX.W movq rax,[r13-0x68] > 0x7f6f3e26fd4b 171 488be5 REX.W movq rsp,rbp ;; debug: > statement 90 > ;; js return > 0x7f6f3e26fd4e 174 5d pop rbp > 0x7f6f3e26fd4f 175 c20800 ret 0x8 > 0x7f6f3e26fd52 178 cc int3 > 0x7f6f3e26fd53 179 cc int3 > 0x7f6f3e26fd54 180 cc int3 > 0x7f6f3e26fd55 181 cc int3 > 0x7f6f3e26fd56 182 cc int3 > 0x7f6f3e26fd57 183 cc int3 > > --- Optimized code --- > kind = OPTIMIZED_FUNCTION > name = loop > stack_slots = 2 > Instructions (size = 234) > 0x7f6f3e2703a0 0 55 push rbp > 0x7f6f3e2703a1 1 4889e5 REX.W movq rbp,rsp > 0x7f6f3e2703a4 4 56 push rsi > 0x7f6f3e2703a5 5 57 push rdi > 0x7f6f3e2703a6 6 4883ec10 REX.W subq rsp,0x10 > 0x7f6f3e2703aa 10 488bc6 REX.W movq rax,rsi > 0x7f6f3e2703ad 13 493b6508 REX.W cmpq rsp,[r13+0x8] > 0x7f6f3e2703b1 17 7305 jnc 24 (0x7f6f3e2703b8) > 0x7f6f3e2703b3 19 e828edfdff call 0x7f6f3e24f0e0 ;; code: STUB, > StackCheckStub, minor: 0 > 0x7f6f3e2703b8 24 488b4627 REX.W movq rax,[rsi+0x27] > 0x7f6f3e2703bc 28 488b4027 REX.W movq rax,[rax+0x27] > 0x7f6f3e2703c0 32 488945e8 REX.W movq [rbp-0x18],rax > 0x7f6f3e2703c4 36 bb00000000 movl rbx,(nil) > 0x7f6f3e2703c9 41 48895de0 REX.W movq [rbp-0x20],rbx > 0x7f6f3e2703cd 45 83fb05 cmpl rbx,0x5 > 0x7f6f3e2703d0 48 0f8d5c000000 jge 146 (0x7f6f3e270432) > 0x7f6f3e2703d6 54 48ba881d7f636f7f0000 REX.W movq rdx,0x7f6f637f1d88 > ;; global property cell > 0x7f6f3e2703e0 64 488b12 REX.W movq rdx,[rdx] > 0x7f6f3e2703e3 67 493b55a0 REX.W cmpq rdx,[r13-0x60] > 0x7f6f3e2703e7 71 0f8456000000 jz 163 (0x7f6f3e270443) > 0x7f6f3e2703ed 77 49ba719295636f7f0000 REX.W movq r10,0x7f6f63959271 > ;; object: 0x7f6f63959271 <JS Function print> > 0x7f6f3e2703f7 87 493bd2 REX.W cmpq rdx,r10 > 0x7f6f3e2703fa 90 0f8550000000 jnz 176 (0x7f6f3e270450) > 0x7f6f3e270400 96 50 push rax > 0x7f6f3e270401 97 488bd3 REX.W movq rdx,rbx > 0x7f6f3e270404 100 48c1e220 REX.W shlq rdx,32 > 0x7f6f3e270408 104 52 push rdx > 0x7f6f3e270409 105 48bf719295636f7f0000 REX.W movq rdi,0x7f6f63959271 > ;; object: 0x7f6f63959271 <JS Function print> > 0x7f6f3e270413 115 b801000000 movl rax,0x1 > 0x7f6f3e270418 120 4c89e1 REX.W movq rcx,r12 > 0x7f6f3e27041b 123 ff5717 call [rdi+0x17] ;; debug: > position 76 > 0x7f6f3e27041e 126 488b75f8 REX.W movq rsi,[rbp-0x8] > 0x7f6f3e270422 130 488b45e0 REX.W movq rax,[rbp-0x20] > 0x7f6f3e270426 134 83c001 addl rax,0x1 > 0x7f6f3e270429 137 488bd8 REX.W movq rbx,rax > 0x7f6f3e27042c 140 488b45e8 REX.W movq rax,[rbp-0x18] > 0x7f6f3e270430 144 eb97 jmp 41 (0x7f6f3e2703c9) > 0x7f6f3e270432 146 48b8b1208f636f7f0000 REX.W movq rax,0x7f6f638f20b1 > ;; object: 0x7f6f638f20b1 <undefined> > 0x7f6f3e27043c 156 488be5 REX.W movq rsp,rbp > 0x7f6f3e27043f 159 5d pop rbp > 0x7f6f3e270440 160 c20800 ret 0x8 > 0x7f6f3e270443 163 49ba4aa0283e6f7f0000 REX.W movq r10,0x7f6f3e28a04a > ;; deoptimization bailout 1 > 0x7f6f3e27044d 173 41ffe2 jmp r10 > 0x7f6f3e270450 176 49ba54a0283e6f7f0000 REX.W movq r10,0x7f6f3e28a054 > ;; deoptimization bailout 2 > 0x7f6f3e27045a 186 41ffe2 jmp r10 > 0x7f6f3e27045d 189 90 nop > 0x7f6f3e27045e 190 90 nop > 0x7f6f3e27045f 191 90 nop > 0x7f6f3e270460 192 90 nop > 0x7f6f3e270461 193 90 nop > 0x7f6f3e270462 194 90 nop > 0x7f6f3e270463 195 90 nop > 0x7f6f3e270464 196 90 nop > 0x7f6f3e270465 197 90 nop > 0x7f6f3e270466 198 90 nop > 0x7f6f3e270467 199 90 nop > 0x7f6f3e270468 200 90 nop > 0x7f6f3e270469 201 90 nop > 0x7f6f3e27046a 202 6690 nop > > It appears that the unoptimized code is emitting two stack limit checks. > One at offset 16 at the function entry (but prior to the loop starting at > offset 32), and another at offset 102, prior to the loop's backward jump. > However, the optimized code is only emitting one stack check at offset 19, > when the loop doesn't appear to start until offset 41. Correct me if I'm > wrong, but the optimized code only appears to be checking the stack limit at > the function entry, not in the loop itself. > > - Kyle > > > > On Wed, Aug 31, 2011 at 3:53 PM, Vyacheslav Egorov <[email protected]> > wrote: >> Hi Kyle, >> >> Optimizing compiler inserts stack checks (HStackCheck instruction) >> explicitly at loop body's entry[1]. >> >> It also does an optimization pass[2] to remove redundant stack checks >> that are dominated by function calls (as functions always does stack >> check in the prologue). >> >> Stack checks are important part of V8's interruption mechanism so both >> compilers emit them to make all loops interruptable. >> >> [1] http://code.google.com/p/v8/source/browse/trunk/src/hydrogen.cc#2823 >> [2] http://code.google.com/p/v8/source/browse/trunk/src/hydrogen.cc#1247 >> >> -- >> Vyacheslav Egorov >> >> >> On Wed, Aug 31, 2011 at 9:27 PM, Kyle <[email protected]> wrote: >>> Hello, >>> >>> Some time ago I noticed that the v8 compile was inserting stack limit >>> checks at the back edges of loops. I later found out that this check >>> was doubling as a preemption mechanism to interrupt potentially long- >>> running code. However, I've noticed that the hydrogen/lithium >>> compiler included with crankshaft does not seem to include these >>> checks. Is there a particular reason for this? Is the previous >>> design for JavaScript preemption no longer being pursued? >>> >>> -- >>> v8-users mailing list >>> [email protected] >>> http://groups.google.com/group/v8-users >>> >> >> -- >> v8-users mailing list >> [email protected] >> http://groups.google.com/group/v8-users > > -- > v8-users mailing list > [email protected] > http://groups.google.com/group/v8-users -- v8-users mailing list [email protected] http://groups.google.com/group/v8-users
