[RFC][PATCH][AArch64] Improve generic branch cost

[Wilco Dijkstra]

Hi,

Recently we've put a lot of effort into improving ifcvt to use CSEL on AArch64.
In  https://gcc.gnu.org/ml/gcc-patches/2015-11/msg01639.html James determined
the best value for AArch64 code generation.  Although this setting is used when
explicitly targeting Cortex cores, it is not otherwise used.  This means by
default GCC will not use (F)CSEL in many common cases.  Most code is built
without -mcpu= and thus doesn't use CSEL like this example from GLIBC:

strtok:
    stp    x29, x30, [sp, -48]!
    add    x29, sp, 0
    stp    x21, x22, [sp, 32]
    mov    x21, x1
    stp    x19, x20, [sp, 16]
    adrp    x22, .LANCHOR0
    mov    x19, x0
    cbz    x0, .L12
.L2:    ldrb    w0, [x19]

.L12:
    ldr    x19, [x22, #:lo12:.LANCHOR0]
    b    .L2

With -mcpu=cortex-a57 GCC generates:

    stp    x29, x30, [sp, -48]!
    cmp    x0, 0
    add    x29, sp, 0
    stp    x21, x22, [sp, 32]
    adrp    x21, .LANCHOR0
    stp    x19, x20, [sp, 16]
    mov    x19, x0
    ldr    x0, [x21, #:lo12:.LANCHOR0]
    csel    x19, x0, x19, eq
    ldrb    w0, [x19]

This is generally faster and smaller.  On one benchmark the new setting fixes a 
regression since GCC6 and improves performance by 49%.  So I propose to change
generic_branch_cost to be the same as cortexa57_branch_cost so that all supported
cores benefit equally from CSEL.  Are there any objections to this?

Wilco


ChangeLog:
2017-03-09  Wilco Dijkstra  <wdijkstr@arm.com>

	* config/aarch64/aarch64.c (generic_branch_cost): Copy cortexa57_branch_cost.
--

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 5870b5e5d7e8e48cf925b3a62030346f041a7fd6..ea16074af86087a6200d9895583e05acf43d90e2 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -377,8 +377,8 @@ static const struct cpu_vector_cost xgene1_vector_cost =
 /* Generic costs for branch instructions.  */
 static const struct cpu_branch_cost generic_branch_cost =
 {
-  2,  /* Predictable.  */
-  2   /* Unpredictable.  */
+  1,  /* Predictable.  */
+  3   /* Unpredictable.  */
 };
 
 /* Branch costs for Cortex-A57.  */