On 03/04/15 09:18, Wilco Dijkstra wrote:
Jeff Law wrote:
On 02/26/15 10:30, Wilco Dijkstra wrote:
Several GCC versions ago a conditional negate optimization was introduced as a
workaround
for
PR45685. However the branchless expansion for conditional negate is extremely
inefficient on
most
targets (5 sequentially dependent instructions rather than 2 on AArch64). Since
the
underlying issue
has been resolved (the example in PR45685 no longer generates a branch on x64),
remove the
workaround so that conditional negates are treated in exactly the same way as
conditional
invert,
add, subtract, and, orr, xor etc. Simple example:
int f(int x) { if (x > 3) x = -x; return x; }
You need to bootstrap and regression test the change before it can be
approved.
You should turn this little example into a testcase. It's fine with me
if this new test is ARM specific.
You should also find a way to change the test gcc.dg/tree-ssa/pr45685.c
in such a way that it ensures there aren't any undesirable branches.
I've got enough history to know this is fixing a regression of sorts for
the ARM platform. So once the issues above are addressed it can go
forward even without a BZ noting the regression.
I updated the x64 testcase to explicitly check for conditional move (which
was simpler than checking for unnecessary branches). Also add a new testcase
for AArch64 to ensure we continue to emit csneg. Bootstrapped & regression
tested on AArch64 and x64.
ChangeLog:
2015-03-04 Wilco Dijkstra <wdijk...@arm.com>
* gcc/tree-ssa-phiopt.c (neg_replacement): Remove.
(tree_ssa_phiopt_worker): Remove negate optimization.
* gcc/testsuite/gcc.dg/tree-ssa/pr45685.c: Update test case
to check for conditional move on x64.
* gcc/testsuite/gcc.target/aarch64/csneg-1.c (test_csneg_cmp):
New test.
Can you move pr45685.c into gcc.target/i386?
I know Richi said next stage1, but given this fixes a performance
regression for ARM and it's reverting rather than adding new code, I
think this is OK for the trunk with the testcase moved.
So, OK with the testcase moved into gcc.target/i386/
jeff
