[LLVMdev] Preserving NSW/NUW bits

[David Majnemer]

So, the trunc makes this transform a bit questionable because you have to
reason about bits in the middle of your value.

Consider the following input values:
%indvars.iv = 0x........7fffffff
%0 = 0x8......1

Consider the following values in the 'before' case:
%2 = 0x........7fffffff
%indvars.iv.next = 0x........80000000
%tmp = 0x7fffffff
%cmp = false

This results in the following values in the 'after' case:
%indvars.iv.next = 0x........80000000
%tmp = 0x80000000
%cmp = true

Regardless of whether or not you propagate nsw/nuw, the transform is
currently unsound.

The transform is sound if %indvars.iv has a sufficiently large number of
signbits.
e.x.
%indvars.iv = sext i31 %invdars.actual to i64

If this precondition holds, any existing nsw or nuw flags should still hold.



On Tue, Sep 2, 2014 at 3:22 PM, Chad Rosier <mcrosier at codeaurora.org> wrote:

> David/All,
> Just a quick question about NSW/NUW bits, if you've got a second.  I
> noticed you've been doing a little work on this as of late.
>
> I have a bit of code that looks like the following:
>
>   %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
>   %2 = add i64 %indvars.iv.next, -1
>   %tmp = trunc i64 %2 to i32
>   %cmp = icmp slt i32 %tmp, %0
>   br i1 %cmp, label %for.body, label %for.end.loopexit
>
> I'm trying to fold the 2nd add instruction into the compare by changing
> the condition from from 'slt' to 'sle':
>
>   %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
>   %tmp = trunc i64 %indvars.iv.next to i32
>   %cmp = icmp sle i32 %tmp, %0
>   br i1 %cmp, label %for.body, label %for.end.loopexit
>
> However, AFAICT the NSW bits must be set.  In what cases can we propagate
> the NSW bit from the first add to the second, if any?  If a case exists,
> can this be handled by the WillNotOverflowSignedAdd function?
>
>  Chad
>
>
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