[PATCH] D55735: [InstSimplify] Simplify saturating add/sub + icmp

[Nikita Popov via Phabricator via llvm-commits]

nikic marked an inline comment as done.
nikic added inline comments.


================
Comment at: lib/Analysis/InstructionSimplify.cpp:2681-2682
+    } else if (match(II.getOperand(1), m_APInt(C))) {
+      if (C->isNegative()) {
+        // ssub.sat(x, -C) produces [SINT_MIN - (-C), SINT_MAX]:
+        Lower = APInt::getSignedMinValue(Width) - *C;
----------------
spatel wrote:
> nikic wrote:
> > spatel wrote:
> > > What happens if C is the signed min value here?
> > > 
> > > ```
> > > define i1 @ssub_icmp_op1_is_min_val(i8 %a) {
> > >   %b = call i8 @llvm.ssub.sat.i8(i8 %a, i8 -128)
> > >   %c = icmp sle i8 %b, -118
> > >   ret i1 %c
> > > }
> > > 
> > > ```
> > This will fold to `ret i1 false`. The computed range here is `[0, SINT_MAX]`. The last test is intended to check this case.
> Ah, sorry I missed that test.
> 
> It's not obvious to me that the result of:
>   %b = call i8 @llvm.ssub.sat.i8(i8 0, i8 -128)
> is "127".
> 
> Is that worth noting here or in the LangRef?
If the semantics aren't clear, it would be good to clarify in LangRef. However, right now I'm not sure I understand where the ambiguity lies. What result would you expect for this operation?


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  https://reviews.llvm.org/D55735/new/

https://reviews.llvm.org/D55735