destructive immediate bitfield insert

[jnk0le]

"place n bits from rs1 into [m+n-1 : m] of rd"

• deprecates insb instruction
• same thing as PULP p.insert
• requires 2 * 5 bits of imm for RV32 and 2 * 6 for RV64
• as P ext is already sourcing 3rd arg from rd (including INSB), it shouldn't be that big of a deal

[jnk0le]

Note that the eqivalent bfp from 0.94 bitmanip is a 3 instruction sequence, 1 of them being loop invariant in equivalent use case.

bitfield inserts are also usefull for uC register acces

// switch PLL (0b10) to HSE (0b01)
RCC->CFGR = (RCC->CFGR & ~RCC_CFGR_SW_Msk) | (RCC_CFGR_SW_HSE);

in this sample 0.94 bfp gives no advantage - andn + or + extra load of RCC_CFGR_SW_Msk solves it in 3 instructions