r187969 - The only useful loop unrolling flag to give realistically is
Hal Finkel
hfinkel at anl.gov
Tue Aug 27 21:46:36 PDT 2013
----- Original Message -----
> ----- Original Message -----
> > ----- Original Message -----
> > > Author: chandlerc
> > > Date: Thu Aug 8 03:34:35 2013
> > > New Revision: 187969
> > >
> > > URL: http://llvm.org/viewvc/llvm-project?rev=187969&view=rev
> > > Log:
> > > The only useful loop unrolling flag to give realistically is
> > > '-fno-unroll-loops'. The option to the backend is even called
> > > 'DisableUnrollLoops'. This is precisely the form that Clang
> > > *didn't*
> > > support.
> >
> > Maybe we should hook up the positive form to enabling the
> > more-aggressive unrolling options: -unroll-allow-partial
> > -unroll-runtime -- doing these kinds of unrolling is not always a
> > win (I think it gives a mean regression on x86_64, but it gives a
> > mean speedup on my A2 cores), but I believe that gcc does
> > equivalent
> > things when -funroll-loops is given.
> >
> > Also, even with this change, -fno-unroll-loops with -O2 or -O3
> > might
> > not really prevent all loop unrolling because the loop vectorizer
> > also unrolls loops based on ILP (and register pressure)
> > considerations. I think that passing -force-vector-unroll=1 would
> > turn this off.
I discussed this loop vectorizer interaction with Nadav, and I implemented this -force-vector-unroll=1 functionality in r189440.
-Hal
>
> Chandler, thoughts?
>
> -Hal
>
> >
> > -Hal
> >
> > > We didn't recognize the flag, we didn't pass it to the CC1
> > > layer, and even if we did we wouldn't use it. Clang only
> > > inspected
> > > the
> > > positive form of the flag, and only did so to enable loop
> > > unrolling
> > > when
> > > the optimization level wasn't high enough. This only occurs for
> > > an
> > > optimization level that even has a chance of running the loop
> > > unroller
> > > when optimizing for size.
> > >
> > > This commit wires up the 'no' variant, and switches the code to
> > > actually
> > > follow the standard flag pattern of using the last flag and
> > > allowing
> > > a flag in either direction to override the default.
> > >
> > > I think this is still wrong. I don't know why we disable the loop
> > > unroller entirely *from Clang* when optimizing for size, as the
> > > loop
> > > unrolling pass *already has special logic* for the case where the
> > > function is attributed as optimized for size! We should really be
> > > trusting that. Maybe in a follow-up patch, I don't really want to
> > > change
> > > behavior here.
> > >
> > > Modified:
> > > cfe/trunk/include/clang/Driver/Options.td
> > > cfe/trunk/lib/Driver/Tools.cpp
> > > cfe/trunk/lib/Frontend/CompilerInvocation.cpp
> > > cfe/trunk/test/Driver/clang_f_opts.c
> > >
> > > Modified: cfe/trunk/include/clang/Driver/Options.td
> > > URL:
> > > http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Driver/Options.td?rev=187969&r1=187968&r2=187969&view=diff
> > > ==============================================================================
> > > --- cfe/trunk/include/clang/Driver/Options.td (original)
> > > +++ cfe/trunk/include/clang/Driver/Options.td Thu Aug 8 03:34:35
> > > 2013
> > > @@ -791,6 +791,8 @@ def ftrap_function_EQ : Joined<["-"], "f
> > > def funit_at_a_time : Flag<["-"], "funit-at-a-time">,
> > > Group<f_Group>;
> > > def funroll_loops : Flag<["-"], "funroll-loops">,
> > > Group<f_Group>,
> > > HelpText<"Turn on loop unroller">, Flags<[CC1Option]>;
> > > +def fno_unroll_loops : Flag<["-"], "fno-unroll-loops">,
> > > Group<f_Group>,
> > > + HelpText<"Turn off loop unroller">, Flags<[CC1Option]>;
> > > def funsigned_bitfields : Flag<["-"], "funsigned-bitfields">,
> > > Group<f_Group>;
> > > def funsigned_char : Flag<["-"], "funsigned-char">,
> > > Group<f_Group>;
> > > def funwind_tables : Flag<["-"], "funwind-tables">,
> > > Group<f_Group>;
> > >
> > > Modified: cfe/trunk/lib/Driver/Tools.cpp
> > > URL:
> > > http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Driver/Tools.cpp?rev=187969&r1=187968&r2=187969&view=diff
> > > ==============================================================================
> > > --- cfe/trunk/lib/Driver/Tools.cpp (original)
> > > +++ cfe/trunk/lib/Driver/Tools.cpp Thu Aug 8 03:34:35 2013
> > > @@ -2977,7 +2977,8 @@ void Clang::ConstructJob(Compilation &C,
> > > CmdArgs.push_back("-fwrapv");
> > > }
> > > Args.AddLastArg(CmdArgs, options::OPT_fwritable_strings);
> > > - Args.AddLastArg(CmdArgs, options::OPT_funroll_loops);
> > > + Args.AddLastArg(CmdArgs, options::OPT_funroll_loops,
> > > + options::OPT_fno_unroll_loops);
> > >
> > > Args.AddLastArg(CmdArgs, options::OPT_pthread);
> > >
> > >
> > > Modified: cfe/trunk/lib/Frontend/CompilerInvocation.cpp
> > > URL:
> > > http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Frontend/CompilerInvocation.cpp?rev=187969&r1=187968&r2=187969&view=diff
> > > ==============================================================================
> > > --- cfe/trunk/lib/Frontend/CompilerInvocation.cpp (original)
> > > +++ cfe/trunk/lib/Frontend/CompilerInvocation.cpp Thu Aug 8
> > > 03:34:35
> > > 2013
> > > @@ -351,8 +351,9 @@ static bool ParseCodeGenArgs(CodeGenOpti
> > > Opts.OptimizeSize = getOptimizationLevelSize(Args);
> > > Opts.SimplifyLibCalls = !(Args.hasArg(OPT_fno_builtin) ||
> > > Args.hasArg(OPT_ffreestanding));
> > > - Opts.UnrollLoops = Args.hasArg(OPT_funroll_loops) ||
> > > - (Opts.OptimizationLevel > 1 &&
> > > !Opts.OptimizeSize);
> > > + Opts.UnrollLoops =
> > > + Args.hasFlag(OPT_funroll_loops, OPT_fno_unroll_loops,
> > > + (Opts.OptimizationLevel > 1 &&
> > > !Opts.OptimizeSize));
> > >
> > > Opts.Autolink = !Args.hasArg(OPT_fno_autolink);
> > > Opts.AsmVerbose = Args.hasArg(OPT_masm_verbose);
> > >
> > > Modified: cfe/trunk/test/Driver/clang_f_opts.c
> > > URL:
> > > http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Driver/clang_f_opts.c?rev=187969&r1=187968&r2=187969&view=diff
> > > ==============================================================================
> > > --- cfe/trunk/test/Driver/clang_f_opts.c (original)
> > > +++ cfe/trunk/test/Driver/clang_f_opts.c Thu Aug 8 03:34:35 2013
> > > @@ -37,6 +37,13 @@
> > > // FP-CONTRACT-FAST-CHECK: -ffp-contract=fast
> > > // FP-CONTRACT-OFF-CHECK: -ffp-contract=off
> > >
> > > +// RUN: %clang -### -S -funroll-loops %s 2>&1 | FileCheck
> > > -check-prefix=CHECK-UNROLL-LOOPS %s
> > > +// RUN: %clang -### -S -fno-unroll-loops %s 2>&1 | FileCheck
> > > -check-prefix=CHECK-NO-UNROLL-LOOPS %s
> > > +// RUN: %clang -### -S -fno-unroll-loops -funroll-loops %s 2>&1
> > > |
> > > FileCheck -check-prefix=CHECK-UNROLL-LOOPS %s
> > > +// RUN: %clang -### -S -funroll-loops -fno-unroll-loops %s 2>&1
> > > |
> > > FileCheck -check-prefix=CHECK-NO-UNROLL-LOOPS %s
> > > +// CHECK-UNROLL-LOOPS: "-funroll-loops"
> > > +// CHECK-NO-UNROLL-LOOPS: "-fno-unroll-loops"
> > > +
> > > // RUN: %clang -### -S -fvectorize %s 2>&1 | FileCheck
> > > -check-prefix=CHECK-VECTORIZE %s
> > > // RUN: %clang -### -S -fno-vectorize -fvectorize %s 2>&1 |
> > > FileCheck -check-prefix=CHECK-VECTORIZE %s
> > > // RUN: %clang -### -S -fno-vectorize %s 2>&1 | FileCheck
> > > -check-prefix=CHECK-NO-VECTORIZE %s
> > >
> > >
> > > _______________________________________________
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> > > cfe-commits at cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
> > >
> >
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> > _______________________________________________
> > cfe-commits mailing list
> > cfe-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
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