[llvm] r186425 - When the inliner merges allocas, it must keep the larger alignment

[Duncan Sands]

Hi Hal,

 > When the inliner merges allocas, it must keep the larger alignment
>
> For safety, the inliner cannot decrease the allignment on an alloca when
> merging it with another.
>
> I've included two variants of the test case for this: one with DataLayout
> available, and one without. When DataLayout is not available, if only one of
> the allocas uses the default alignment (getAlignment() == 0), then they cannot
> be safely merged.

> --- llvm/trunk/lib/Transforms/IPO/Inliner.cpp (original)
> +++ llvm/trunk/lib/Transforms/IPO/Inliner.cpp Tue Jul 16 12:10:55 2013
> @@ -189,6 +190,14 @@ static bool InlineCallIfPossible(CallSit
>       bool MergedAwayAlloca = false;
>       for (unsigned i = 0, e = AllocasForType.size(); i != e; ++i) {
>         AllocaInst *AvailableAlloca = AllocasForType[i];
> +
> +      unsigned Align1 = AI->getAlignment(),
> +               Align2 = AvailableAlloca->getAlignment();
> +      // If we don't have data layout information, and only one alloca is using
> +      // the target default, then we can't safely merge them because we can't
> +      // pick the greater alignment.
> +      if (!TD && (!Align1 || !Align2) && Align1 != Align2)
> +        continue;
>
>         // The available alloca has to be in the right function, not in some other
>         // function in this SCC.
> @@ -206,6 +215,11 @@ static bool InlineCallIfPossible(CallSit
>                      << *AvailableAlloca << '\n');
>
>         AI->replaceAllUsesWith(AvailableAlloca);
> +
> +      if (Align1 > Align2 || (!Align1 && TD &&
> +          TD->getABITypeAlignment(AI->getAllocatedType()) > Align2))
> +        AvailableAlloca->setAlignment(Align1);

If Align1 is 1 and Align2 is zero then you set the alignment to Align1 but
Align2 might represent a larger alignment.  How about something like this:

   if (Align1 != Align2) {
      if (!Align1 || !Align2) {
        assert(TD && "Should have bailed out earlier!");
        unsigned TypeAlign = TD->getABITypeAlignment(AI->getAllocatedType());
        if (!Align1)
          Align1 = TypeAlign;
        if (!Align2)
          Align2 = TypeAlign;
      }

      if (Align1 > Align2)
        AvailableAlloca->setAlignment(Align1);
   }

Ciao, Duncan.

> +
>         AI->eraseFromParent();
>         MergedAwayAlloca = true;
>         ++NumMergedAllocas;