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

[Andrea Di Biagio]

Hi Quentin,


================
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:
> 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?

http://reviews.llvm.org/D8605

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