[ping^2][PATCH][2 of 2] RTL expansion for zero sign extension elimination with VRP

[Kugan]

Hi Eric,

Can you please help to review the general idea and this patch for  zero 
sign extension elimination with VRP?


Thanks,
Kugan

On 17/06/13 11:01, Kugan wrote:
Can you please help to review this patch? Richard reviewed the original
patch and asked it to be split into two parts. Also, he wanted a review
from RTL maintainers for the RTL changes.

Thanks,
Kugan

On 03/06/13 11:46, Kugan wrote:
Hi,

This patch  removes some of the redundant sign/zero extensions using
value range information during RTL expansion.

When GIMPLE_ASSIGN stmts with LHS type smaller than word is expanded to
RTL, if we can prove that RHS expression value can always fit in LHS
type and there is no sign conversion, truncation and extension to fit
the type is redundant. For a SUBREG_PROMOTED_VAR_P, Subreg and Zero/sign
extensions are therefore redundant.

For example, when an expression is evaluated and it's value is assigned
to variable of type short, the generated RTL would look something like
the following.

(set (reg:SI 110)
                  (zero_extend:SI (subreg:HI (reg:SI 117) 0)))

However, if during value range propagation, if we can say for certain
that the value of the expression which is present in register 117 is
within the limits of short and there is no sign conversion, we do not
need to perform the subreg and zero_extend; instead we can generate the
following RTl.

(set (reg:SI 110)
                  (reg:SI 117)))

Same could be done for other assign statements.

This patch is based on the earlier attempt posted in
http://gcc.gnu.org/ml/gcc-patches/2013-05/msg00610.html and addresses
the review comments of  Richard Biener. I am post-processing the
expand_expr_real_2 output in expand_gimple_stmt though. Reason for this
is that I would like to process all the possible assignment stmts, not
just  CASE_CONVERT case and/or the REDUCE_BITFIELD.

This change along with expansion improve the geomean of spec2k int
benchmark with ref by about ~3.5% on an arm chromebook.

Tested  on X86_64 and ARM.

I would like review comments on this.

Thanks,
Kugan


+2013-06-03  Kugan Vivekanandarajah  <kug...@linaro.org>
+
+    * gcc/dojump.c (do_compare_and_jump): generates rtl without
+    zero/sign extension if redundant.
+    * gcc/cfgexpand.c (expand_gimple_stmt_1): Likewise.
+    * gcc/gimple.c (gimple_assign_is_zero_sign_ext_redundant) : New
+    function.
+    * gcc/gimple.h (gimple_assign_is_zero_sign_ext_redundant) : New
+    function definition.
+