Re: S/390: Fix string-opt-17.c testsuite failure

[Roger Sayle <roger at eyesopen dot com>]

Hi Ulrich,

> Incidentally, this fixes the string-opt-17.c test case, which failed
> on 64-bit because it tried to build a constant using a TImode
> multiplication.  Not only do we not support that, but even if we
> would, I wouldn't consider replacing a 16-byte memset with a
> __multi3 call to be an optimization ...

Its strange that the s390 backend would want to call __multi3.
The multiplication is by a simple integer constant, and on most
targets this should be turned into a sequence of shifts and adds.
So GCC should generate a sequence of eight shift and add instructions
to build the value to write in a TI mode register, and then a single
store of that TI register to memory (which should be faster than 16
consecutive bytes stores).  On some platforms with fast hardware
multiply, the optimal sequence may actually include a real multiply.
But is sounds like the RTX_COST of a TI mode multiplication on
S/390 is underestimated.  The function call to __multi3 must
be more expensive than the shift-add sequence that would
otherwise be prefered by GCC.

memset16:
	reg &= 0xff;
	reg |= reg<<8;
	reg |= reg<<16;
	reg |= reg<<32;
	reg |= reg<<64;
	*ptr = reg;

GCC only considers the TImode operations as the backend claims to
support TImode reads and writes to and from memory.  Most architectures
allow at most SImode sequences, performing memset 4 bytes at a time.

Anyway, I hope this explains the motivation for the "optimization".

Roger
--
Roger Sayle,                         E-mail: roger@eyesopen.com
OpenEye Scientific Software,         WWW: http://www.eyesopen.com/
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