[LLVMdev] Embedding cpu and feature strings into IR and enabling switching subtarget on a per function basis

[Eric Christopher]

Hi Akira,

A bit of a follow up to my earlier email with some review on the patches
themselves. Some of them I think are good and some I think need some
thought.

+  Options.CPUStr = MCPU;
+  Options.FeaturesStr = MAttrs;

I'd definitely like to avoid having the CPU and Feature string on
TargetOptions. Ideally the only things that are going to be on there are
global options that can't be anywhere else - and I'd really like to
investigate module flags here as well. I know we've had some discussion on
that, and I've got my own opinions, but in general anything that's per
function needs to be kept on the functions and anything that's general
should be kept on the module.

+  template <typename STC> const STC &getSubtarget(const Function *F) const
{
+    return *static_cast<const STC*>(getSubtargetImpl(*F));

Right. Though you missed getSubtargetImpl above, etc. A lot of the passes
and backends use it rather than the other way around. I'm pulling them out
and more help is definitely good. As I said in my first email perhaps the
TTI->FTTI transition is a good place to start working here without having
to worry much about redoing huge swaths of llvm.

-    : Fn(F), Target(TM), STI(TM.getSubtargetImpl()), Ctx(mmi.getContext()),
+    : Fn(F), Target(TM), STI(TM.getSubtargetImpl(*F)),
Ctx(mmi.getContext()),

This is obviously fine, but I've been avoiding this so that we don't have
partial implementations in flight. I.e. if you do this then code that looks
like what we want in the future will start being ... halfway code generated.

+  // Find the first function defined in the module and copy the cpu and
+  // feature string to Options.

Very much against this.

 // FIXME: This should stop caching the target machine as soon as
 // we can remove resetOperationActions et al.
-X86TargetLowering::X86TargetLowering(const X86TargetMachine &TM)
-    : TargetLowering(TM) {
-  Subtarget = &TM.getSubtarget<X86Subtarget>();
+X86TargetLowering::X86TargetLowering(const X86TargetMachine &TM,
+                                     const X86Subtarget *ST)
+    : TargetLowering(TM), Subtarget(ST) {

If we get rid of resetOperationActions then we don't need to do this and it
only takes the X86Subtarget here.

-    : LLVMTargetMachine(T, TT, CPU, FS, Options, RM, CM, OL),
+    : LLVMTargetMachine(T, TT, Options.CPUStr.val(),
Options.FeaturesStr.val(),
+                        Options, RM, CM, OL),
       TLOF(createTLOF(Triple(getTargetTriple()))),
       DL(computeDataLayout(Triple(TT))),
-      Subtarget(TT, CPU, FS, *this, Options.StackAlignmentOverride) {
+      Subtarget(TT, Options.CPUStr, Options.FeaturesStr, *this,
+                Options.StackAlignmentOverride) {

Again, I don't think this is the right way to go. The default CPU and FS
can quite easily just be what's default in the module, there's no need to
do this.

Anyhow, it's definitely the direction I've been headed, I think there's
some disconnect on how various things related to options should go and
there's a lot of work that can be done in the meantime. I'll see what I can
do to write up a more detailed plan of attack and start listing good places
to start working on this (or complicated and hard ones if you'd like, there
are a few of those left as well!)

Thanks!

-eric


On Mon Jan 26 2015 at 4:28:41 PM Akira Hatanaka <ahatanak at gmail.com> wrote:

> I've been investigating what is needed to ensure command line options are
> passed to the backend codegen passes during LTO and enable compiling
> different functions in a module with different command line options (see
> the links below for previous discussions).
>
> http://thread.gmane.org/gmane.comp.compilers.llvm.devel/78855
> http://thread.gmane.org/gmane.comp.compilers.llvm.devel/80456
>
> The command line options I'm currently looking into are "-target-cpu" and
> "-target-feature" and I would like to get feedback about the approach I've
> taken (patches attached).
>
>


> The attached patches make the following changes:
>
> - In TargetMachine::getSubtarget(const Function*) and MachineFunction's
> constructor, use per-function subtarget object instead of TargetMachine's
> (module-level) subtarget object. This allows passes like selection dag to
> switch the target on a per-function basis.
>
> - Define class TargetOptions::Option, which records whether an option has
> appeared on the command line along with the option's value. Long term, this
> might not be the best solution and I expect it will be modified or replaced
> when the new command line infrastructure becomes available.
>
> - Fix X86's subtarget lookup to override the function attributes if the
> corresponding options were specified on the command line.
>
> - FIx clang to embed "-target-cpu" and "-target-feature" attributes in the
> IR.
>
> I've tested the changes I made and confirmed that target options such as
> "-mavx2" don't get dropped during LTO and are honored by backend codegen
> passes.
>
> This is my plan for the remaining tasks:
>
> 1. FIx other in-tree targets and other code-gen passes that are still
> using TargetMachine's subtarget where the per-function subtarget should be
> used.
>
> 2. Fix TargetTransformInfo to compute the various code-gen costs
> accurately when subtarget is switched on a per-function basis. One way to
> do this is to make the pointer or reference to the Function object
> available to the various subclasses of TargetTransformInfo by defining the
> necessary functions in FunctionTargetTransformInfo (similar to the changes
> made in r218004). However, passes like Inliner that are not function passes
> cannot access FunctionTargetTransformInfo, so it has to be done in a
> different way.
>
> 3. Forbid inlining functions that have incompatible cpu and feature
> attributes. It seems the simplest approach is to allow inlining only if the
> cpu and feature attributes match exactly, but it's also possible to relax
> this restriction.
>
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