Xref: utzoo comp.lang.c:10264 comp.lang.fortran:678 Path: utzoo!utgpu!water!watmath!clyde!bellcore!faline!thumper!ulysses!andante!princeton!udel!gatech!ncar!ames!pasteur!ucbvax!decwrl!sun!dgh!dgh From: dgh@dgh.UUCP Newsgroups: comp.lang.c,comp.lang.fortran Subject: no noalias not negligible - a difference between C and Fortran - long Message-ID: <54080@sun.uucp> Date: 21 May 88 01:36:43 GMT Sender: news@sun.uucp Lines: 151 Posted: Fri May 20 21:36:43 1988 noalias may be non-negotiable, but it may be non-negligible, as I found out somewhat to my surprise this week. At various times I've needed a version of The Linpack Benchmark written in C, usually because a Fortran compiler was not available; The Linpack Benchmark is very useful for lots of surprising purposes, like debugging caches and virtual memory systems. Of course, The Linpack Benchmark is by definition written entirely in Fortran, so for benchmarking purposes a C translation is more or less useless. This despite an observation by one of my colleagues, viewing the C version, that it appeared to still be written in Fortran. But faithful preservation of Fortran semantics, including memory access patterns, was one of the goals of the translation. My new manager wanted to tackle a small technical job keep in shape, and so I suggested this translation. It was not quite as small a job as we expected, but eventually she got it to produce identical numerical results as the Fortran version, but she never could get comparable performance. The results for double precision linpack on Sun-4 using SunOS 4.0 and Fortran 1.1 were: Rolled Source Unrolled Source Fortran 1080 KFLOPS 875 KFLOPS C 850 KFLOPS 875 KFLOPS Why was C slower than Fortran in the rolled case? It turns out that almost all The Linpack Benchmark does is execute a short subroutine of the following simplified Fortran forms: subroutine daxpy(n,da,dx,dy) doubleprecision dx(1),dy(1),da integer i,n do 30 i = 1,n,4 dy(i) = dy(i) + da*dx(i) dy(i+1) = dy(i+1) + da*dx(i+1) dy(i+2) = dy(i+2) + da*dx(i+2) dy(i+3) = dy(i+3) + da*dx(i+3) 30 continue return end OR subroutine daxpy(n,da,dx,dy) doubleprecision dx(1),dy(1),da integer i,n do 30 i = 1,n dy(i) = dy(i) + da*dx(i) 30 continue return end The first of these is the standard UNROLLED form of the program; the second is a questionably legal modification called the ROLLED form. The original Fortran was written with unrolled source because that generated more efficient code ten years ago when most compilers didn't unroll loops for you automatically. Nowadays many Fortran compilers, including Sun's, can get better performance by unrolling the loops themselves than by attempting to figure out unrolled source code. Most of the benefit of loop unrolling in high-performance systems derives from the possibilities it opens up for instruction scheduling across independent iterations. The current multiplication can overlap the previous addition, or current computation can overlap previous stores and subsequent loads; what is worthwhile varies among implementations. The corresponding rolled C code could be written with a for loop daxpy(n, da, dx, dy ) double dx[], dy[], da; int n; { int i; for (i = 0; i < n; i++) { dy[i] = dy[i] + da * dx[i]; } } but Sun compilers catch a red herring: for loops are compiled into two basic blocks and the unroller won't unroll loops with more than one basic block. So to bypass that issue, rewrite a little bit more in the form daxpy(n, da, dx, dy ) double dx[], dy[], da; int n; { int i; i = 0; do { dy[i] = dy[i] + da * dx[i]; } while (i++ < n); } In this form the optimizer still won't unroll it, and consequently the performance is less than Fortran. If the source form is unrolled, however, the optimizer can't do as much with Fortran, and C performance is the same. Investigation revealed that the reason had to do with noalias: all Fortran pointer variables (which happen to be exactly the set of procedure parameters) are defined by the Fortran standard to be "noalias", meaning a compiler may optimize code based on the assumption that the pointers never reference the same memory. Alleged Fortran programs which break under such optimization are declared by the Fortran standard to be non-standard. Very neat. C, in contrast, has other kinds of pointer variables than procedure parameters, and many people believe that a global decree of the Fortran type would break a lot of existing C programs. So the default is that optimization must assume that any two pointers may be pointing to the same thing unless it can prove otherwise. For a while X3J11 had a local "noalias" attribute that you could attach to pointers, but later recanted in consideration to assertions like 1) nobody had done it before, which is probably true, 2) nobody could agree on exactly what it meant, which appeared to be true, and 3) optimizing compilers should be able to figure out if aliasing exists, which is definitely false in a separate compilation environment (unless you want the linker to recompile everything, in which case the linker is the compiler, and you're back to no separate compilation). Anyway there is no portable way in draft ANSI C to say "this pointers are guaranteed to have no aliases". Thus the first part of the C compiler does NOT tell the optimizer that any pointers are guaranteed unaliased; the optimizer won't unroll anything if there are potential aliasing problems: you don't dare load dx[i+1] before you store dy[i] if there is any danger that they point to the same place. The Fortran compiler need have no such qualms. What is to be done? I submitted extensive commentary to X3J11 during the last public review period about numerical issues, but didn't mention noalias because it was such a hot potato and I didn't think it mattered much, not having investigated the possibilities. Even if noalias could be proved to be unquestionably a good idea, I doubt X3J11 would want to change its draft again, since such proofs seem so easy to overturn. Perhaps what will happen is that high-performance C compilers will adopt the questionable CDC/Cray Fortran practice of providing "unsafe" optimization levels that, for instance, assume all pointers are unaliased. David Hough dhough@sun.com na.hough@na-net.stanford.edu {ucbvax,decvax,decwrl,seismo}!sun!dhough