On 1/13/2019 4:08 AM, David Crayford wrote:
On 13/01/2019 7:06 pm, Tony Thigpen wrote:
I have seen some reports that current C compilers, which understand
the z-hardware pipeline, can actually produce object that is faster
running than an assembler. Mainly because no sane assembler
programmer would produce great pipe-line code because it would be
un-maintanable.
It's well established that that's been true for a well over a decade
now. Not just C but all compilers including COBOL which got a new
optimizer a few releases back.
Far, far less true now than it used to be.
Back in the old days, things ran a lot faster if you interleaved
unrelated things in an "unfriendly" way. For example, this code fragment:
| LGF R0,Field1 Increment Field1
| AGHI R0,1 (same)
| ST R0,Field1 (same)
| LGF R0,Field2 Increment Field2
| AGHI R0,1 (same)
| ST R0,Field2 (same)
| LGF R0,Field3 Increment Field3
| AGHI R0,1 (same)
| ST R0,Field3 (same)
| LGF R0,Field4 Increment Field4
| AGHI R0,1 (same)
| ST R0,Field4 (same)
ran much faster when coded this way (which is not how a programmer would
usually write things):
| LGF R0,Field1 Increment Field1
| LGF R1,Field2 Increment Field2
| LGF R2,Field3 Increment Field3
| LGF R3,Field4 Increment Field4
| AGHI R0,1 (same)
| AGHI R1,1 (same)
| AGHI R2,1 (same)
| AGHI R3,1 (same)
| ST R0,Field1 (same)
| ST R1,Field2 (same)
| ST R2,Field3 (same)
| ST R3,Field5 (same)
But once OOO execution came on the scene with z196, you could get the
same enhanced performance from this easy-to-code and easy-to-read version:
| LGF R0,Field1 Increment Field1
| AGHI R0,1 (same)
| ST R0,Field1 (same)
| LGF R1,Field2 Increment Field2
| AGHI R1,1 (same)
| ST R1,Field2 (same)
| LGF R2,Field3 Increment Field3
| AGHI R2,1 (same)
| ST R2,Field3 (same)
| LGF R3,Field4 Increment Field4
| AGHI R3,1 (same)
| ST R3,Field4 (same)
These days, many performance improvements are realized by the compiler
using newer instructions that replace older ones. For example, on z10
and higher, this very same code can be replaced with:
| ASI Field1,1 Increment Field1
| ASI Field2,1 Increment Field1
| ASI Field3,1 Increment Field1
| ASI Field4,1 Increment Field1
Of course, an HLASM programmer can do exactly the same thing. But
changing old code to use new instructions requires relatively-expensive
programmer resources whereas simply recompiling programs targeting a new
machine is a relatively-inexpensive proposition.
--
Phoenix Software International
Edward E. Jaffe
831 Parkview Drive North
El Segundo, CA 90245
https://www.phoenixsoftware.com/
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