> On Mar 26, 2018, at 2:26 PM, Ivan Gerasimov <ivan.gerasi...@oracle.com> wrote: > > Thank you Andrew for looking into this! > > > On 3/24/18 4:13 AM, Andrew Haley wrote: >> On 03/20/2018 05:20 PM, Ivan Gerasimov wrote: >>> I tried to run it, but the numbers are non-distinguishable for non-zero >>> arguments. >>> And my variant performs slightly better with zero argument. >>> So, I think it's reasonable to keep the variant with the ternary operator. >> I am suspicious of this argument. Did you look at the generated code? >> >> I get >> >> cbnz w10, 0x000003ffa8202384 >> mov w0, wzr >> >> for the zero test and >> >> cbz w10, 0x000003ffa81d228c >> clz w11, w10 >> orr w10, wzr, #0x80000000 >> lsr w0, w10, w11 ;*iushr >> >> for 42. >> >> The branch at the start of both versions goes to a deoptimize trap. >> We don't want deoptimize traps if we can avoid them, so the branchless >> version is better IMO. > This looks persuasive, so let's go ahead with the branchless variant! >
+1 In my experience with nano-benchamarks like this it's often informative to look at the generated code. This is even easier now that JMH supports a dtrace ASM profiler on the Mac: http://mail.openjdk.java.net/pipermail/jmh-dev/2018-January/002686.html <http://mail.openjdk.java.net/pipermail/jmh-dev/2018-January/002686.html> (YMMV, I ran into some issues on the mac whereby the spawned dtrace stopped logging output and needed to poked with a signal into action and dump the rest of its output. Not had time to investigate in detail and report back on this.) Paul.
