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" To: 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)