Re: [PATCH] Implement ‘optnone’ attribute semantics

[Bill Wendling]

On Nov 13, 2013, at 4:09 PM, Robinson, Paul <Paul_Robinson at playstation.sony.com> wrote:

> I am energized by positive discussions of the 'optnone' attribute
> during the dev meeting last week, so here we go.
> 
> Some time ago Andrea Di Biagio committed a patch to define an 'optnone'
> attribute in the LLVM IR; he has since passed the torch to me on this
> one, so I'm attaching 3 patches to implement the semantics we want.
> 
> While the patches all address semantics of 'optnone' I should point
> out that (I'm pretty sure) they are independent of each other and
> don't have to be reviewed sequentially, or anything like that.
> 
> Patch #3 deals with the codegen stuff.  Basically, an 'optnone'
> function will be treated as if it were compiled at -O0; this is a
> little easier to accomplish than it is for IR-level passes.
> 

You changes in Patch #3 won’t work. You’re changing the TargetOptions settings, but that violates what the back-end is expecting. For instance, the UnsafeFPMath is used during the creation of the back-end to register which actions to perform for which operations. E.g., in X86ISelLowering.cpp X86TargetLowering::resetOperationActions():

    if (!TM.Options.UnsafeFPMath) {
      setOperationAction(ISD::FSIN   , MVT::f64, Expand);
      setOperationAction(ISD::FCOS   , MVT::f64, Expand);
      setOperationAction(ISD::FSINCOS, MVT::f64, Expand);
    }

Is it possible to do FastISel only as an initial stage?

Also, use early exits whenever possible. E.g., this

+    ~OptLevelChanger() {
+      if (IS.OptLevel != SavedOptLevel) {
+        DEBUG(dbgs() << "\nRestoring optimization level for Function "
+              << IS.MF->getFunction()->getName() << "\n");
+        DEBUG(dbgs() << "\tBefore: -O" << IS.OptLevel
+              << " ; After: -O" << SavedOptLevel << "\n");
+        IS.OptLevel = SavedOptLevel;
+        IS.TM.setOptLevel(SavedOptLevel);
+      }
+    }

could be this:

+    ~OptLevelChanger() {
+      if (IS.OptLevel == SavedOptLevel)
+        return
+      DEBUG(dbgs() << "\nRestoring optimization level for Function "
+            << IS.MF->getFunction()->getName() << "\n");
+      DEBUG(dbgs() << "\tBefore: -O" << IS.OptLevel
+            << " ; After: -O" << SavedOptLevel << "\n");
+      IS.OptLevel = SavedOptLevel;
+      IS.TM.setOptLevel(SavedOptLevel);
+    }

-bw

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