firstbit (sm5 - asm)
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 |