Edit

firstbit (sm5 - asm)

  • Article

Finds the first bit set in a number, either from LSB or MSB.

firstbit{_hi|_lo|_shi} dest[.mask], src0[.swizzle]
Item Description
dest
 [in] The integer position of the first bit set in src0 starting from the LSB for firstbit_lo or MSB for firstbit_hi.
src0
 [in] The input integer.

Remarks

This operation returns the integer position of the first bit set in the 32-bit input starting from the LSB for firstbit_lo or MSB for firstbit_hi. For example firstbit_lo on 0x00000001 returns 0. firstbit_hi on 0x10000000 returns 3.

firstbit_shi (s for signed) returns the first 0 from the MSB if the number is negative; otherwise it returns the first 1 from the MSB.

All variants of the instruction return ~0 (0xffffffff in 32-bit register) if no match is found.

Use this instruction to quickly enumerate set bits in a bitfield, or find the largest power of 2 in a number.

This instruction applies to the following shader stages:

Vertex Hull Domain Geometry Pixel Compute
X X X X X X

Mimimum Shader Model

This instruction is supported in the following shader models:

Shader Model Supported
Shader Model 5 yes
Shader Model 4.1 no
Shader Model 4 no
Shader Model 3 (DirectX HLSL) no
Shader Model 2 (DirectX HLSL) no
Shader Model 1 (DirectX HLSL) no

Shader Model 5 Assembly (DirectX HLSL)