[PATCH] D136080: [flang] Add -ffp-contract option processing

Fri Oct 21 05:00:13 PDT 2022

tblah added inline comments.

================
Comment at: flang/include/flang/Frontend/LangOptions.h:29
+
+    // Enable the floating point pragma
+    FPM_On,
----------------
vzakhari wrote:
> 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`?
Thanks for your feedback.

I've updated my patch. Now flang only supports `off` and `fast`. The other two map to `fast` and we default to `off`.

gfortran seems to default to `fast`:
```
-ffp-contract=style

    -ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. This is currently not implemented and treated equal to -ffp-contract=off.

    The default is -ffp-contract=fast.
```

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

================
Comment at: flang/test/Driver/driver-help-hidden.f90:34
 ! CHECK-NEXT: Use <value> as character line width in fixed mode
+! CHECK-NEXT: -ffp-contract=<value> 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) | fast-honor-pragmas (fuses across statements unless diectated by pragmas). Default is 'fast' for CUDA, 'fast-honor-pragmas' for HIP, and 'on' otherwise.
 ! CHECK-NEXT: -ffree-form            Process source files in free form
----------------
awarzynski wrote:
> tblah wrote:
> > vzakhari wrote:
> > > Is it easy to emit a different help message for Flang to say that there are only two modes for Fortran?
> > @awarzynski tells me there is no way to do this short of having separate `Options.td` for flang and clang. Once we have settled on which arguments to support, I will update the shared help text to mention flang.
> Both `clang -help` and `flang-new -help` must be 100% correct. As this help text is not valid in the case of LLVM Flang, it needs to be updated accordingly. 
> 
> As @tblah points out, there's no straightforward mechanism for having a custom help texts for `clang` and `flang-new` in Clang's driver library ATM. But I think that this can be achieved even without creating a separate "Options.td" file. One would have to define a new tablegen record in "Options.td". That would be a separate patch though, probably accompanied by an RFC.
> 
> There's a different solution too. Note that currently the definition uses the `HelpText` field. However, you could use [[ https://github.com/llvm/llvm-project/blob/b1d7a95e4e4a2b57cbe02636bbe357dc48d615c5/clang/include/clang/Driver/Options.td#L81-L82 | DocBrief ]] instead (which we used to solve a similar issue with [[ https://github.com/llvm/llvm-project/blob/b1d7a95e4e4a2b57cbe02636bbe357dc48d615c5/clang/include/clang/Driver/Options.td#L696-L704 | -I ]]). That's what I suggest that you do. Basically, copy the contents of `HelpText` for `-ffp-contract` into a `DocBrief` field (we don't use this field in Flang and it should probably be renamed as `DocBriefClang`). `HelpText` should be replaced with something brief that applies both to Clang and Flang.
> 
> Btw, what's the help-text/spelling in `gfortran`?
Thanks for the feedback.

I have added a mention of flang to the old help, which has been moved to `DocBrief`. The new `HelpText` just mentions which options are supported by each frontend without explaining them.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D136080/new/

https://reviews.llvm.org/D136080