We're doing some performance work on our assembler code and one of my
colleagues ran the following test which was surprising. Unconditional
branching can add significant overhead. I always believed that
conditional branches were expensive because the branch predictor needed
to do more work and unconditional branches were easy to predict. Does
anybody have an explanation for this. Our machine is z114. It appears
that it's even worse on a z13.
Here's the code.
I wrote a simple program - it tight loops 1 billion times
L R4,=A(1*1000*1000*1000)
LTR R4,R4
J LOOP
*
LOOP DS 0D .LOOP START
B NEXT
NEXT JCT R4,LOOP
The loop starts with a branch ... I tested it twice - when the CC is
matched (branch happens) and when it is not matched (falls through)
1. When the CC is matched and branching happens, CPU TIME=2.94 seconds
2. When the CC is not matched the code falls through, CPU TIME=1.69
seconds - a reduction of 42%
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