Bringing this over from gcc-patches@
Jakub Jelinek wrote:
On Fri, Dec 09, 2011 at 01:50:37PM +0100, Georg-Johann Lay wrote:
No, not OK.
This leads to unacceptable code for devices that cannot shift easily like,
e.g.AVR. This target can only shift by 1 and shifts with big offsets have
to be performed by means of a loop at runtime.
Andrew's patch only restored what GCC has been doing before.
If this is too expensive on AVR, you should just arrange in the backend to
combine that
((x>>C)&1) != 0
into more efficient code sequence,
Would you please outline how to do that?
Suppose the following C code:
char test_bit (long long x)
{
if (x & 0x400)
return 1;
else
return 0;
}
Notice that with the patch, optabs generate __lshrdi3 libgcc call.
So you will have to get rid of the libgcc call.
Without the patch, the generated code is 3 instructions:
test_bit:
ldi r24,lo8(1) ; 22 *movqi/2 [length = 1]
sbrs r19,2 ; 32 *sbrx_branchqi [length = 2]
ldi r24,lo8(0) ; 33 *movqi/1 [length = 1]
.L2:
ret ; 61 return [length = 1]
Situations for other modes are not better.
otherwise people who write
if ((x >> 18) & 1)
bar ();
in the source will get suboptimal code.
Jakub
Well, that just depends on what people you care for and what people you
ignore...
I still don't see why optimizing code for platform X is allowed to lead
to code bloat for platform Y.
The problem is:
* There is no canonical representation for "extract bit" or "move bit"
or "test bit" in the tree passes.
* tree -> RTL lowering does not care for RTX costs for the
different ways "testing a bit" etc. can be represented.
If there was a canonical representation of these operations, a backend
wouldn't even notice if the tree passes chose a different, more
convenient canonicalization.
The very problem is obviously that various ways to extract/test/move a
bit are not worked out during tree -> RTL expansion and that writing RTX
costs is paper waste.
What you are saying is: "Just change your backend if the code gets bad."
Maybe next week there is yet another change for platform Z and operation
Q. Again, the backend needs to be reworked and expand to be reverse
engineered to reveal what is this week's favourite style of expanding Q?
Really, if this is how architectural goal or development roadmap looks
like, then contributing to GCC is just waste of time.
Johann
