Programming Forums > Application Development > Assembly

earl · Jul 11th, 2005, 6:53 PM

With GCC, the '&' character can be used to specify an "earlyclobber" operand of asm modules. I was reading a piece of code that used this for an output constraint, and got into reading more about what it's for. I'm pretty confused by the generic definition that seems to be everywhere I look, however..

Quote:

Means (in a particular alternative) that this operand is an earlyclobber operand, which is modified before the instruction is finished using the input operands. Therefore, this operand may not lie in a register that is used as an input operand or as part of any memory address.

& applies only to the alternative in which it is written. In constraints with multiple alternatives, sometimes one alternative requires & while others do not. See, for example, the movdf insn of the 68000.

An input operand can be tied to an earlyclobber operand if its only use as an input occurs before the early result is written. Adding alternatives of this form often allows GCC to produce better code when only some of the inputs can be affected by the earlyclobber.

& does not obviate the need to write =

Could anybody explain this more clearly? I understand the concept of a clobber list, which is separate from the specific constraints, but I'm confused as to how this applies to a specific operand.

DaWei · Jul 11th, 2005, 8:29 PM

I personally consider that "feature" of gcc to be harder and more confusing than just writing the dam' assembly code, so I don't use it. I suspect the early clobber thing is where an intermediate result would be stored, so you wouldn't want to use it for anything that has other uses upcoming, like one of the inputs.

Jul 11th, 2005, 8:29 PM	#2
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	I personally consider that "feature" of gcc to be harder and more confusing than just writing the dam' assembly code, so I don't use it. I suspect the early clobber thing is where an intermediate result would be stored, so you wouldn't want to use it for anything that has other uses upcoming, like one of the inputs. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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Programming Forums > Application Development > Assembly

Inline asm - earlyclobber operand