[instcombine]: Slice a big load in two loads when the element are next to each other in memory.

[Sean Silva]

On Mon, Aug 19, 2013 at 8:35 PM, Quentin Colombet <qcolombet at apple.com>wrote:

> Hi,
>
> Here is a patch that implements the slicing of a load into two smaller
> loads when the elements are next to each other in memory.
> The original motivation as well as the discussion of why we came up with
> this patch can be found here:
> http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-August/064769.html
>
>
> ** Motivation **
>
> To give a bit of context, the motivation was to get rid of truncate and
> shift right instructions that get in the way of paired load or floating
> point load.
> E.g.,
> Consider the following example:
> struct Complex {
> float real;
> float imm;
> };
>
> When accessing a complex, llvm was generating a 64-bits load and the imm
> field was obtained by a trunc(lshr) sequence, resulting in poor code
> generation, at least for x86.
>
>
Interesting. I saw something similar
("extract-fields-from-chunk-of-bits") while
investigating the issue reported in PR16739 (the second IR example in the
bug report is an example), except that the trunc/lshr's were being created
by GVN and not by SROA. <http://llvm.org/bugs/show_bug.cgi?id=16739>

-- Sean Silva
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