[PATCH] [X86][FastIsel] Teach how to select vector load instructions.

[hfinkel at anl.gov]

================
Comment at: lib/Target/X86/X86FastISel.cpp:1099
@@ +1098,3 @@
+  if (Alignment == 0) // Ensure that codegen never sees alignment 0
+    Alignment = ABIAlignment;
+  bool Aligned = Alignment >= ABIAlignment;
----------------
qcolombet wrote:
> andreadb wrote:
> > qcolombet wrote:
> > > By default, don’t we assume stuff are unaligned, i.e., Alignment = 1?
> > That's what I thought originally. However, when testing this patch, I noticed that without fast-isel I was always getting aligned load instructions.
> > 
> > Example:
> > ```
> > define <4 x float> @foo(<4 x float>* %V) {
> > entry:
> >   %0 = load <4 x float>, <4 x float>* %V
> >   ret <4 x float> %0
> > }
> > ```
> > without -fast-isel, llc -mattr=+sse2 generates a vmovaps.
> > 
> > As far as I understand, if a load doesn't specify the alignment, then the backend assumes the 'ABIAlignment' based on the value type.
> > 
> > This also matches the behavior of method 'X86FastISel::X86SelectStore'. At around line 934, we check for the alignment of the store in input; if a store node doesn't specify any alignment (i.e. Alignment is 0), then FastISel implicitly sets Alignment to the 'ABIAlignment'.
> > For the record: the code between lines 1096 and 1099 is mostly copy-pasted from that method.
> > 
> > In general I don't have a strong opinion on this: I can change line 1099 so that we propagate value 1 to Alignment. However, if we do this, then for consistency, we should  probably also change how we check the alignment of store instructions in 'X86SelectStore'.  What do you think?
> It seems we do the same for SelectionDAG, at least at some places, unlike what I thought.
> May be worth bringing that up on the dev mailing list.
> 
> Anyhow, this part looks good as it is consistent with what exists.
The general convention all throughout LLVM is that Alignment == 0 means to default to the required ABI alignment. You need Alignment == 1 to get completely unaligned.

http://reviews.llvm.org/D8605

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