Use movss to implement insertelement when elements = 4 and index = 0.

Use movss to implement insertelement when elements = 4 and index = 0.

This avoids using a pair of shufps instructions as the previous lowering
was doing.  Instead, we use movss to copy the element to be inserted
into the lower 32 bits of the destination.

Define InstX8632Movss as a Binop, the class to which it properly
belongs.

BUG=none
R=jvoung@chromium.org, stichnot@chromium.org

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-subzero.git;a=commit;h=cfe5146

[wala, 2014-07-25 02:28:20]

Note that while movss was declared in the wrong category of instructions (it was
previously a Unaryop and this change makes it a Binop), the previous code was
never using it in a way that would expose this bug.

The only previous usage was "movss dest, src" which was using movss in a way to
"define" the lower 32 bits of dest.

The need to declare movss as a binary instruction occurs if the upper 96 bits of
dest matter, which is true in the case that we're using movss to implement
insertelement.

[jvoung (off chromium), 2014-07-25 15:58:29]

lgtm

[Jim Stichnoth, 2014-07-25 20:07:12]

lgtm

https://codereview.chromium.org/412353005/diff/1/src/IceInstX8632.h
File src/IceInstX8632.h (right):

https://codereview.chromium.org/412353005/diff/1/src/IceInstX8632.h#newcode588
src/IceInstX8632.h:588: typedef InstX8632Binop<InstX8632::Movss> InstX8632Movss;
Could you add a TODO here, or somewhere appropriate, that the Dest (and hence
Src0) operand of the movss instruction should have a vector type?  Also, that
Dest should be a physical register.

The rationale is that (correct me if I'm wrong) the movss instruction is really
only a binary op when used with a vector instruction like insertelement,
otherwise it's more like mov/movsx/movzx.

[wala, 2014-07-25 22:57:41]

https://codereview.chromium.org/412353005/diff/1/src/IceInstX8632.h
File src/IceInstX8632.h (right):

https://codereview.chromium.org/412353005/diff/1/src/IceInstX8632.h#newcode588
src/IceInstX8632.h:588: typedef InstX8632Binop<InstX8632::Movss> InstX8632Movss;
On 2014/07/25 20:07:12, stichnot wrote:
> Could you add a TODO here, or somewhere appropriate, that the Dest (and hence
> Src0) operand of the movss instruction should have a vector type?  Also, that
> Dest should be a physical register.
> 
> The rationale is that (correct me if I'm wrong) the movss instruction is
really
> only a binary op when used with a vector instruction like insertelement,
> otherwise it's more like mov/movsx/movzx.

Done.

Per our discussion, movss is a binary op only when both source and dest operands
are registers. Any current use of _movss as a non binary op should be made to
use _mov eventually.

[wala, 2014-07-25 22:58:00]

Committed patchset #2 manually as rcfe5146 (presubmit successful).