sof 2003/01/22 11:15:23 PST Modified files: src prelude.h static.c storage.c storage.h subst.c Log: Stephen Milborrow's optimisations to whatIs() -- enabled by defining FAST_WHATIS to 1 (which it is now by default.) Here's the original e-mail describing the changes made (and their effect): From: "Stephen Milborrow" <milbo@icon.co.za> To: <hugs-users@haskell.org> Subject: A Faster whatIs Date: Sat, 30 Nov 2002 11:10:47 +0200 Hello everyone: I made some changes to the Hugs sources which give small runtime speed gains. The results are in the table below. The gains are minor, but I thought some people in the group might be interested. WHATIS WHATIS1 FHEAP Speed Increase (percent) gamteb 8 9 5 parser 6 8 gcfail prolog 9 9 2 queens 10 13 4 Extra BSS memory (kBytes) 24 24 0 Nbr source lines changed 50 144 41 ---Notes On The Table The table headings WHATIS, WHATIS1, and FHEAP refer to three different sets of changes. WHATIS and WHATIS1 are described below. FHEAP was an experiment that simply used a fixed size heap (an array instead of a malloc) and is impractical because it rules out use of the Hugs -h flag. The (motley) collection of test programs are from nofib. I timed runhugs for each program without command line flags, except that for queens I used -h1000000. Different programs would probably give different results: the usual caveats apply. These programs were just the nofib ones I had at hand. I compiled with MSC 5.0 Service Pack 3 using modified Nov 2002 Hugs sources. I ran the timing tests on a Windows 98 machine. ---Strategy I wanted to see what speed gains could be achieved by taking a worm's eye view of the code, with no changes to fundamental algorithms. I also wanted to make changes that would be limited to just a few places in the code -- a minimal force approach. To start off, I ran the MSC profiler. This confirmed that whatIs() is a candidate for optimization, as already noted in Mark's Gopher implementation document and in comments in storage.c. But the profiler showed a few other hotspots too. (An interesting thing to do is sort the profiler results on execution line-count, which immediately tells you which are the most executed lines in the program.) ---WHATIS Change To reduce the time spent in whatIs, I created a byte array whatCode of whatIs codes. I then created a whatIs macro which replaces the whatIs function: #define whatIs(c) (isPair(c)? \ (isTag(fst(c)) ? fst(c) : AP ) : \ whatCode[c]) Negative indexing into whatCode is prevented by the isPair. To keep the size of whatCode reasonable, I had to reduce the range of unboxed ints -- the bigger the range, the bigger the whatCode array. I settled on a range of 2048 i.e. ints between -1023 and 1024 are unboxed, all others are boxed. The extra boxing will actually slow down execution of certain Haskell programs, but as far as I can tell the majority of real Hugs programs would be unaffected. Increasing this range to, say, 10 000 would increase memory usage by 10 000 bytes -- nothing really when you consider that the memory footprint of winhugs is about 10 MBytes. This change yields the speed improvements under the WHATIS column in the table. ---WHATIS1 Change IsTag is defined as #define isTag(c) (TAGMIN<=(c) && (c)<SPECMIN) I reduced the cost of IsTag slightly by changing defines in storage.h so that only box-cell-tags are in the range 1 to 0x7f. (I shifted special cell values down to start at 0x80). I then defined variants for IsTag and friends: #define isTag1(c) (((c) & TAG_MASK) == 0) #define whatIs1(c) (isPair(c)? \ (isTag1(fst(c))? fst(c) : AP ) : \ whatCode[c]) #define isAp1(c) (isPair(c) && !isTag1(fst(c))) whatIs1 is a faster version of whatIs, with the "bug" that it will return the wrong value if fst(c) is NIL. When used in several places as a replacement for whatIs, it gains us a few speed percentage points as shown in the WHATIS1 column above. WHATIS1 sits on top of WHATIS so the gains attributable to WHATIS1 alone are the differences between the percentages in the two columns. ---Another Candidate Another candidate for this kind of optimization is the line in eval(): if (!isCfun(n) && (ar=name(n).arity)<=(sp-base)) {... This is one of the most executed lines in the entire program. If the stored arity of Cfun's was offset by a largish number, say 10000, then the above test against isCfun wouldn't be needed. This change would introduce inefficiencies elsewhere (we would have to un-offset the stored arity before using it elsewhere) but the net effect would probably be a speed gain. I shied away from this change because it would require ubiquitous (though easy) code changes. ---Final Comments Optimizing code written by Mark Jones is a challenge. It becomes a little less daunting if we change the rules by allowing ourselves to waste some memory in the pursuit of speed, and to introduce some ugliness into the code. Even so, the speed improvements I got were small. If anyone is interested, I would be happy to send the sources. They are modified Nov 2002 sources with all the changes demarcated by #define's. Six files changed. I would be interested to know what results these changes give on other machines. --- Stephen Milborrow Revision Changes Path 1.40 +7 -2 hugs98/src/prelude.h 1.134 +7 -2 hugs98/src/static.c 1.59 +38 -2 hugs98/src/storage.c 1.48 +86 -2 hugs98/src/storage.h 1.26 +6 -2 hugs98/src/subst.c