[LLVMdev] sign extensions, SCEVs, and wrap flags

[Andrew Trick]

On Sep 19, 2012, at 2:18 PM, Preston Briggs <preston.briggs at gmail.com> wrote:

> On Tue, Sep 18, 2012 at 10:59 PM, Andrew Trick <atrick at apple.com> wrote:
> 
> On Sep 18, 2012, at 8:21 PM, Preston Briggs <preston.briggs at gmail.com> wrote:
> 
>> Given the following SCEV,
>> 
>> (sext i32 {2,+,1}<nw><%for.body> to i64)
>> 
>> from the following C source,
>> 
>> void strong3(int *A, int *B, int n) {
>>   for (int i = 0; i < n; i++) {
>>     A[i + 2] = i;
>>     ...
>>   }
>> }
>> 
>> Since the No-Wrap flag is set on the addrec, can't I safely rewrite it as
>> 
>> {2,+,1}<nw><%for.body>
>> 
>> If I can, why isn't the SCEV package simplifying things for me?
> 
> The short answer is that SCEV is terrible at preserving NSW flags. I personally don't believe they belong in SCEV but the merits of making any design change here are dubious.
> 
> To understand one example of SCEV dropping NSW, see createSCEV for Instruction::Add. Synopsis: your add is not a "basic induction variable" so its NSW flag does not bound the number of loop iterations. We only know that the add's original IR users expect NSW. There could be other IR adds with the same expression, but without the NSW flag. SCEV doesn't know anything about acyclic control flow or IR users, so it must drop the flags.
> 
> I would try hard not to rely on NSW flags on arbitrary SCEVs. I would first find the phi or basic induction variable before checking the recurrence's NSW flag. Or, better yet, only rely on SCEVAddRec's NW (no self-wrap flag) rather than NSW. Notice that the NW is preserved in your add's recurrence!
> 
> -Andy
> 
> 
> OK.  I think...
> Basically, I'm trying to understand how two subscripts relate to one another. When I find sign and zero extensions, life gets confusing. In an effort to keep life simple, I begin by walking though the expressions, trying to eliminate extensions where it won't change the answer. For example, I think
>> (sext i32 {2,+,1}<nw><%for.body> to i64)
> is the same as
>> {2,+,1}<nw><%for.body>
> 
> right?
> 
> Mechanically, when I see an sext over an addrec and the addrec has the NW flag, then I can rewrite is as an addrec<nw> with the base and step extended. In this case, the base and step are constants, which are particularly easy.
> 
> On the other hand, if the addrec is missing the NW flag, I'd be making a mistake.
> 
> In a similar vein, it seems plausible that I can rewrite a sign-extend over an add (or multiply), as long as the add (multiply) has the NSW flag, right? Same for zero-extend, over add with NUW flag.

Sorry, I probably led you astray. No-self-wrap is useful for determining trip count, but does not mean that sign/zero extension can be hoisted.

But if you run your analysis after -indvars, the sign-extension should be removed if possible. The algorithm walks the derived induction variables specifically looking for add nsw/nuw and replacing only those IR users with a wide induction variable. See GetExtendedOperandRecurrence.

If you have a situation where the sign extends cannot removed, then you may need your own IR-level analysis to determine whether you can ignore them. Otherwise, you may run into trouble operating on SCEVs that contain sext/zext/truncs. Are all your array indices uniformly sign-extended? I don't know if this is a good idea, but why can't you consider the sext operand the array index rather than the gep operand? If you prove that the narrow indices are disjoint, then the extended indices must be disjoint. SCEV operations should work fine on the narrow indices which shouldn't have any impure type casting operations.

This can all be avoided by limiting your optimization to code that uses pointer-size loop counters!

-Andy

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