CODEWITHSUNDEEP
javascriptoptimizationv8profilerdeoptimization
Paul Borza
Paul Borza

Reputation: 191

Chrome's V8 won't optimize function because of "tagged-to-i: not a heap number"

I have a function that's supposed to round X to the nearest multiple of A.

Here's the code in JavaScript:

var round = function (x, a) {
  "use strict";

  if (typeof a === "undefined") {
    return Math.round(x);
  }

  var half = a / 2,
      sign = this.sign(x),
      absolute = Math.abs(x),
      factor = absolute / a|0,
      remainder = absolute % a;

  return sign * a * (factor + (remainder < half ? 0 : 1));
};

When I do a --trace_deopt --code_comments in Chrome's V8 I get this message in the logs:

[deoptimizing (DEOPT eager): begin 0x988464b35b1 round (opt #13) @13, FP to SP delta: 24]
 591             ;;; deoptimize at <0:70499> tagged-to-i: not a heap number
 592   translating round => node=67, height=48
 593     0x7fff571f2ed0: [top + 96] <- 0x11b5fb674cf9 ; rdi 0x11b5fb674cf9 <JS Object>
 594     0x7fff571f2ec8: [top + 88] <- 0x2459b11f1d89 ; r8 0x2459b11f1d89 <Number: 45>
 595     0x7fff571f2ec0: [top + 80] <- 0x2d00000000 ; [sp + 40] 45
 596     0x7fff571f2eb8: [top + 72] <- 0xbef1cdacfe5 ; caller's pc
 597     0x7fff571f2eb0: [top + 64] <- 0x7fff571f3090 ; caller's fp
 598     0x7fff571f2ea8: [top + 56] <- 0x11b5fb674d61 ; [sp + 0] 0x11b5fb674d61 <FixedArray[133]>
 599     0x7fff571f2ea8: [top + 56] <- 0x11b5fb674d61; context
 600     0x7fff571f2ea0: [top + 48] <- 0x988464b35b1; function
 601     0x7fff571f2e98: [top + 40] <- 22 ; rcx (smi)
 602     0x7fff571f2e90: [top + 32] <- 0x34565d404121 <undefined> ; literal
 603     0x7fff571f2e88: [top + 24] <- 0x34565d404121 <undefined> ; literal
 604     0x7fff571f2e80: [top + 16] <- 0x34565d404121 <undefined> ; literal
 605     0x7fff571f2e78: [top + 8] <- 0x34565d404121 <undefined> ; literal
 606     0x7fff571f2e70: [top + 0] <- 1 ; rsi (smi)
 607 [deoptimizing (eager): end 0x988464b35b1 round @13 => node=67, pc=0xbef1cd97be6, state=TOS_REG, alignment=no padding, took 0.084 ms]
 608 [removing optimized code for: round]
 609 [evicting entry from optimizing code map (notify deoptimized) for 0x34565d492be1 <SharedFunctionInfo round>]

Does anyone know what tagged-to-i: not a heap number mean?

Thanks in advance!

Upvotes: 2

Views: 710

Answers (1)

Vyacheslav Egorov
Vyacheslav Egorov

Reputation: 10492

tagged-to-i: not a heap number essentially means an V8 is attempting to convert a value that is not a primitive number to an integer.

Originally I thought that the first argument (x) from the translation frame is a Number object because it is printed as <Number: 45>, but as it turns out V8 prints HeapNumber objects this way[1]. Under normal circumstances an attempt to unbox a heap number should not cause not a heap number deoptimization. So it's either a deopt due to different variable - or deoptimization reason is not accurate. 

;; this
0x7fff571f2ed0: [top + 96] <- 0x11b5fb674cf9 ; rdi 0x11b5fb674cf9 <JS Object>
;; x
0x7fff571f2ec8: [top + 88] <- 0x2459b11f1d89 ; r8 0x2459b11f1d89 <Number: 45>
;; a
0x7fff571f2ec0: [top + 80] <- 0x2d00000000 ; [sp + 40] 45

[1] https://github.com/v8/v8-git-mirror/blob/4f9193e047d50b1ffbca95e8185576af82c722b3/src/objects.cc#L1546-L1551

Upvotes: 2

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