Regression with r200947: [CodeGenPrepare] Move away sign extensions that get in the way of addressing mode.

[Tom Stellard]

On Wed, Feb 12, 2014 at 01:43:25PM -0800, Quentin Colombet wrote:
> Hi Tom,
> 
> I think you find a bug in TargetLoweringBase::isLegalAddressingMode.
> 
> Indeed, TargetLoweringBase::isLegalAddressingMode (which is what it is called in your case), says that 3*r + r is a valid addressing mode.
> Basically, this returns true for every scale that is not 0, 1, and 2. I believe it misses a default case in the switch statement that return false.
>
> Alternatively, you could have overloaded this function for your target, to be sure it accepts only what it should.
> 
> Assuming you fixes this problem, the promotion would still be performed because in that case the transformation is neutral (i.e., we do not create new instructions).
> The difference is: instead of selecting:
> %in + %sext
> we would select:
> %in + %promoted_mul
> 
> That said, the mul 64bit is not a legal operation on your platform (nor is mul i32), thus, I could drop the change if we did not fold the instruction into the addressing mode when the operation is not legal.
> 
> What do you think?
> 

I think it makes sense to avoid the promotion if the result can't be
folded into the addressing mode.  How will you determine whether or not an
operation is legal?

-Tom

> Thanks for the test case.
> -Quentin
> 
> On Feb 12, 2014, at 12:58 PM, Quentin Colombet <qcolombet at apple.com> wrote:
> 
> > Sure, I’m looking into it.
> > 
> > -Quentin
> > 
> > On Feb 12, 2014, at 12:03 PM, Tom Stellard <tom at stellard.net> wrote:
> > 
> >> Hi Quentin,
> >> 
> >> I've discovered a regression with this commit, please see the attached test case.
> >> In this case, CodeGenPrepare is promoting the mul in a sext + mul pattern even
> >> though the addressing mode it is creating isn't legal.
> >> 
> >> One interesting thing about this test case is that if you remove the
> >> nsw from the mul instruction, then the incorrect transform does not
> >> take place.  I'm not sure why this matters.  Would you mind taking a look?
> >> 
> >> Thanks,
> >> Tom
> >> <codegen-prepare-addrmode-sext.ll>
> > 
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