Comment #2 on issue 2205 by [email protected]: MathFloorOfDiv
optimization is very fragile
http://code.google.com/p/v8/issues/detail?id=2205
One more data point. For the function like this:
function foo(x, y) {
return Math.floor(~~x / ~~y);
}
We produce:
;;; <@18,#26> tagged-to-i
0x26e2c92c 44 a801 test al,0x1
0x26e2c92e 46 0f8598000000 jnz 204 (0x26e2c9cc)
0x26e2c934 52 d1f8 sar eax,1
;;; <@19,#26> gap
0x26e2c936 54 8b5d08 mov ebx,[ebp+0x8]
;;; <@20,#27> tagged-to-i
0x26e2c939 57 f6c301 test_b bl,0x1
0x26e2c93c 60 0f85cf000000 jnz 273 (0x26e2ca11)
0x26e2c942 66 d1fb sar ebx,1
;;; <@22,#18> div-i
0x26e2c944 68 85db test ebx,ebx
0x26e2c946 70 0f8474010000 jz 448 (0x26e2cac0)
0x26e2c94c 76 85c0 test eax,eax
0x26e2c94e 78 7508 jnz 88 (0x26e2c958)
0x26e2c950 80 85db test ebx,ebx
0x26e2c952 82 0f8868010000 js 448 (0x26e2cac0)
0x26e2c958 88 3d00000080 cmp eax, 0x80000000
0x26e2c95d 93 7509 jnz 104 (0x26e2c968)
0x26e2c95f 95 83fbff cmp ebx,0xff
0x26e2c962 98 0f8458010000 jz 448 (0x26e2cac0)
0x26e2c968 104 99 cdq
0x26e2c969 105 f7fb idiv ebx
0x26e2c96b 107 85d2 test edx,edx
0x26e2c96d 109 0f854d010000 jnz 448 (0x26e2cac0)
;;; <@24,#20> check-non-smi
0x26e2c973 115 f7c101000000 test ecx,0x1
0x26e2c979 121 0f8446010000 jz 453 (0x26e2cac5)
;;; <@26,#21> check-maps
0x26e2c97f 127 8179ff39dcf03d cmp [ecx+0xff],0x3df0dc39 ;; object:
0x3df0dc39 <Map(elements=3)>
0x26e2c986 134 0f853e010000 jnz 458 (0x26e2caca)
;;; <@28,#28> int32-to-double
0x26e2c98c 140 f20f2ac8 cvtsi2sd xmm1,eax
;;; <@30,#22> math-floor
0x26e2c990 144 0f57c0 xorps xmm0,xmm0
0x26e2c993 147 660f2ec8 ucomisd xmm1,xmm0
0x26e2c997 151 750d jnz 166 (0x26e2c9a6)
0x26e2c999 153 660f50d1 movmskpd edx,xmm1
0x26e2c99d 157 f6c201 test_b dl,0x1
0x26e2c9a0 160 0f8529010000 jnz 463 (0x26e2cacf)
0x26e2c9a6 166 660f3a0bc109 roundsd xmm0,xmm1,9
0x26e2c9ac 172 f20f2cd0 cvttsd2si edx,xmm0
0x26e2c9b0 176 81fa00000080 cmp edx,0x80000000
0x26e2c9b6 182 0f8413010000 jz 463 (0x26e2cacf)
;;; <@32,#29> number-tag-i
0x26e2c9bc 188 03d2 add edx,edx
0x26e2c9be 190 0f8095000000 jo 345 (0x26e2ca59)
That's a lot of redundant code.
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