----- Original Message -----
From: "Phil Smith III" <[email protected]>
To: <[email protected]>
Sent: Friday, November 12, 2021 10:58 AM
Subject: Curious compiler optimization
IBM XLC on z/OS 2.4, under USS.
With NOOPT:
* int var = 0;
LA r4,0
LR r0,r4
ST r0,var(,r13,160)
That's an interesting way to clear a fullword, no?
Maybe the compiler, seeing an initialization, establishes zero in register 0
and keeps it there. When it needs zero, it merely copies it into anther
register that it has allocated?
Anyway, it doesn't really need to use Register 4.
And I'm unclear what
var(,r13,160)
is supposed to be-the actual generated opcodes are
5000 D0A0
Which makes sense. Feels like there's an extra comma in there and an extra
offset. Maybe this is the compiler's way of saying "This is tinkering with
var, which is at offset 160"? I.e., it explicitly mentions both the variable
name and the 160 for readability?
With OPT(3):
* int var = 0;
MVHI var(r13,160),H'0'
LA r0,0
Saner, but still a bit curious-halfword?? Maybe it's so clever it's reusing
something that it knows is already half cleared!?! The LA r0,0 is in
preparation for using it in a subsequent instruction, though that doesn't
explain the redundant register tinkering in the NOOPT version.
None of this matters-it all works fine-but it struck me as odd.
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