On 05/03/2012 08:33 AM, William J. Schmidt wrote:
This patch was posted for comment back in February during stage 4. It
addresses a performance issue noted in the EEMBC routelookup benchmark
on a common idiom:
if (...)
x = y->left;
else
x = y->right;
If the two loads can be hoisted out of the if/else, the if/else can be
replaced by a conditional move instruction on architectures that support
one. Because this speculates one of the loads, the patch constrains the
optimization to avoid introducing page faults.
Bootstrapped and regression tested on powerpc-unknown-linux-gnu with no
new failures. The patch provides significant improvement to the
routelookup benchmark, and is neutral on SPEC cpu2000/cpu2006.
One question is what optimization level should be required for this.
Because of the speculation, -O3 might be in order. I don't believe
-Ofast is required as there is no potential correctness issue involved.
Right now the patch doesn't check the optimization level (like the rest
of the phi-opt transforms), which is likely a poor choice.
Doesn't this need to be conditionalized on the memory model that's
currently active?
jeff
