that can be done. But for 4.1 and newer, there is a 'Z' constraint. It does not allow "updating" addresses, but does allow both indexed and offset addresses. However, the only allowed constant offset is 0. We can then use the undocumented 'y' operand modifier, which causes gcc to convert "0(reg)" into the equivilient "0,reg" format that can be used with stwbrx. This brings us the to problem with the BE version. In this case, the "stw" instruction does have both indexed and non-indexed versions. The final asm ends up looking like this: asm("sync; stw%U0%X0 %1,%0" : "=m" (*addr) : "r" (val), "r" (addr)); The undocumented codes "%U0" and "%0X" will generate a 'u' if the memory reference should be an auto-updating one, and an 'x' if the memory reference is indexed, respectively. The third operand is unused, it's just there because asm the code is reused from the LE version. However, gcc does not know this, and generates unnecessary code to stick addr in a register! To use the example from the LE version, gcc will generate "add 11,4,3; stwx 9,4,3". It is able to use the indexed address "4,3" for the "stwx", but still thinks it needs to put 4+3 into register 11, which will never be used. This also ends up happening a lot for the offset addressing mode, where common code like this: out_be32(&device_registers->some_register, data); uses an instruction like "stw 9, 42(3)", where register 3 has the pointer device_registers and 42 is the offset of some_register in that structure. gcc will be forced to generate the unnecessary instruction "addi 11, 3, 42" to put the address into a single (unused) register. The in_* versions end up having these exact same problems as well. Signed-off-by: Trent Piepho CC: Benjamin Herrenschmidt CC: Andreas Schwab Signed-off-by: Paul Mackerras Ë