vzakhari added inline comments.
================ Comment at: flang/include/flang/Frontend/LangOptions.h:29 + + // Enable the floating point pragma + FPM_On, ---------------- tblah wrote: > vzakhari wrote: > > tblah wrote: > > > awarzynski wrote: > > > > What are these pragmas? Perhaps you can add a test that would include > > > > them? > > > I copied this comment from clang. I believe the pragma is > > > ``` > > > #pragma clang fp contract(fast) > > > ``` > > > > > > See > > > https://clang.llvm.org/docs/LanguageExtensions.html#extensions-to-specify-floating-point-flags > > > > > > This patch only adds support for argument processing, so I can't test for > > > the pragmas. > > I do not think we should blindly copy this from clang. I believe > > `-ffp-contract=on` is there to do the contraction complying to the > > language standard - but Fortran standard says nothing about contraction. I > > am also not aware about a Fortran compiler that supports directives related > > to contraction, so `fast-honor-pragmas` does not seem to be applicable as > > well. Basically, we end up with just `off` and `fast`. > > > > Now, it may be reasonable to support the same `-ffp-contract=` arguments so > > that users can apply the same options sets for C/C++ and Fortran > > compilations. If we want to do this, we need to map `on` and > > `fast-honor-pragmas` to something, e.g. `fast`. A driver warning (not an > > error) may be useful to make the option's behavior clear when `on` and > > `fast-honor-pragmas` are passed. > From the clang help text: > ``` > Form fused FP ops (e.g. FMAs): > fast (fuses across statements disregarding pragmas) > | on (only fuses in the same statement unless dictated by pragmas) > | off (never fuses) > Default is 'on' > ``` > > So if we removed `on` and set the default to `off` we would no longer fuse > within the same statement by default. > > Classic-flang seems to support `on`, `off` and `fast`: > https://github.com/flang-compiler/flang/blob/master/test/llvm_ir_correct/fma.f90 Not talking about defaults just yet, I think Flang cannot currently support the `on` mode as documented. I do not have the latest classic flang readily availalbe, but I am curious what it will generate for this example: ``` function fn(x,y,z) real :: x,y,z fn = x * y fn = fn + z end function ``` With a very old classic flang I get just `fast` math flags on the multiple and add instructions, which is obviously not what `on` is supposed to do: ``` $ flang -target aarch64-linux-gnu -O1 -c -S -emit-llvm -ffp-contract=on fma.f90 $ cat fma.ll %4 = fmul fast float %3, %1, !dbg !10 %5 = bitcast i64* %z to float*, !dbg !11 %6 = load float, float* %5, align 4, !dbg !11 %7 = fadd fast float %6, %4, !dbg !11 ``` Maybe the latest classic flang does support it properly, e.g. it only contracts operations from the same statement. But I do not see a way to support this in Flang right now, so documenting the `on` mode as it is in clang seems confusing. We can still support `on` in the Flang option, but I think we need to issue a warning saying that it defaults to something else, e.g. to `fast`. If mapping `on` to `fast` is not appropriate to some users, then they will have to explicitly specify `-ffp-contract=off` for Fortran compilations in their build system. I am also curious what `fuses in the same statement` means for Fortran. For example: ``` x1 = DOT_PRODUCT(x2, x3)+x4*x5+x6 ``` If Fortran processor decides to implement `DOT_PRODUCT` as inline multiply+add loop, does `-ffp-contract=on` apply to them or it only applies to `x4*x5+x6`? CHANGES SINCE LAST ACTION https://reviews.llvm.org/D136080/new/ https://reviews.llvm.org/D136080 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
