[Patch combine] Don't create vector mode ZERO_EXTEND from subregs

[James Greenhalgh]

Hi,

In simplify_set we try transforming the paradoxical subreg expression:

  (set FOO (subreg:M (mem:N BAR) 0))

in to:

  (set FOO (zero_extend:M (mem:N BAR)))

However, this code does not consider the case where M is a vector
mode, allowing it to construct (for example):

  (zero_extend:V4SI (mem:SI))

This would clearly have the wrong semantics, but fortunately we fail long
before then in expand_compound_operation. As we really don't want a vector
zero_extend of a scalar value.

We need to explicitly reject vector modes from this transformation.

This fixes a failure I'm seeing on a branch in which I'm trying to
tackle some performance regressions, so I have no live testcase for
this, but it is wrong by observation.

Tested on aarch64-none-elf and bootstrapped on aarch64-none-linux-gnu with
no issues.

OK?

Thanks,
James

---
2017-12-11  James Greenhalgh  <james.greenhalgh@arm.com>

	* combine.c (simplify_set): Do not transform subregs to zero_extends
	if the destination mode is a vector mode.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0001-Patch-combine-Don-t-create-vector-mode-ZERO_EXTEND-f.patch
Type: text/x-patch
Size: 789 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/gcc-patches/attachments/20171211/4de17c3e/attachment.bin>