ASM to C: Bit Scan Reverse - most significant bit

[STARGÅTE]

Dear all,

how can I translate this ASM code (search the most significant bit) to the C-backend?

Code: Select all

Procedure.i MSB(Quad.q)
	If Quad = 0
		ProcedureReturn -1
	Else
		! MOV rax, [p.v_Quad]
		! BSR rax, rax
		ProcedureReturn
	EndIf
EndProcedure

Debug MSB(%0)
Debug MSB(%1000)
Debug MSB(%10001000)
Debug MSB(%100010001000)

[mk-soft]

Code: Select all

Procedure.i MSB(Quad.q)
	If Quad = 0
		ProcedureReturn -1
	Else
	  CompilerIf #PB_Compiler_Backend = #PB_Backend_Asm
  		! MOV rax, [p.v_Quad]
  		! BSR rax, rax
  		ProcedureReturn
  	CompilerElse
  	  Protected r1
  	  ! v_r1 = __builtin_clzll(v_quad);
  	  ProcedureReturn 63 - r1
  	CompilerEndIf
  EndIf
EndProcedure

Debug MSB(%0)
Debug MSB(%1000)
Debug MSB(%10001000)
Debug MSB(%100010001000)

Link: https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

[STARGÅTE]

Thanks, also for the link to further contents.