------- Additional Comments From schlie at comcast dot net 2004-11-17 07:21 ------- Subject: Re: 3.4.3 ~6x+ performance regression vs 3.3.1
> From: dmixm at marine dot febras dot ru <[EMAIL PROTECTED]> > But foo_ll (shift loop with count 62!) and foo_l have remained on old - > through shift of the left argument. It would seem in general that if GCC in addition to transforming: ((size)x & (size)<pow-of-2-value>)) => (size)(x >> <log2-value>) it also then transformed all RSHIFT expressions of the form: (size)(x >> <value>) => (size-lsw)((subreg:(size-lsw) x:size lsw)) >> (<value>-(word-size*lsw))) [where lsw is the lest significant pow-of-2 word which remains significant i.e lsw = 0 implies all words remain significant, lsw = 1 implies the precision of the result may be reduced by one word, etc.] It would enable for a hypothetical 8-bit word-wide machine: (4-word-type)(X & 0x04000000) => (4-word-type)(X >> 26) => (1-word-type)((subreg:1-word-type X:4-word-type 3) >> 2) Or for a 16-bit word wide machine: (or analogously for even wider machines) (2-word-type)(X & 0x04000000) => (2-word-type)(X >> 26) => (1-word-type)((subreg:1-word-type X:2-word-type 1) >> 10) Which both enables the use of the resulting demoted precision value to be expressed more optimally as an argument in subsequent expressions, and to more optimally generate code on less-then-word-wide target machines. Furthermore, the subreg representation of demoted expression values, could also enable the potential further optimization of sign/zero extended values, by allowing the detection of only sign or zero extended bits remaining significant: (4-word-type)((char)X & (long)0x04000000) => (4-word-type)((long<-char)X >> 26) => (1-word-type)((subreg:char ((long<-char)x):long 3) >> 2) => (1-word-type)((subreg:char (sign x):char 3) >> 2) => (1-word-type)(sign x) Or something like that possibly. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18424
