| commit | 38c4a7370670e6b73cbae192c7d3079c90203832 | [log] [tgz] |
|---|---|---|
| author | Michael Munday <mike.munday@ibm.com> | Thu Aug 15 21:53:37 2019 +0100 |
| committer | Michael Munday <mike.munday@ibm.com> | Wed Oct 09 15:04:59 2019 +0000 |
| tree | 0ed95e4710b6cb9d1f0d40a28b62b8d81eb4dd20 | |
| parent | ff86ce13b69f427248203493d669a730988f882c [diff] |
cmd/asm: add s390x branch-on-count instructions
The branch-on-count instructions on s390x decrement the input
register and then compare its value to 0. If not equal the branch
is taken.
These instructions are useful for implementing loops with a set
number of iterations (which might be in a register).
For example, this for loop:
for i := 0; i < n; i++ {
... // i is not used or modified in the loop
}
Could be implemented using this assembly:
MOVD Rn, Ri
loop:
...
BRCTG Ri, loop
Note that i will count down from n in the assembly whereas in the
original for loop it counted up to n which is why we can't use i
in the loop.
These instructions will only be used in hand-written codegen and
assembly for now since SSA blocks cannot currently modify values.
We could look into this in the future though.
Change-Id: Iaab93b8aa2699513b825439b8ea20d8fe2ea1ee6
Reviewed-on: https://go-review.googlesource.com/c/go/+/199977
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
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