[cfe-dev] NSW and ExtLdPromotion()

[Jonathan Roelofs]

On 8/10/15 7:53 PM, Lawrence wrote:
> Hi, All:
>
> I have a testcase which produced incorrect result, it’s caused by the
> combination of nsw flag and ExtLdPromotion, I am leaning to say Clang
> set nsw flag incorrectly, but please let me know if I was wrong.

nsw is set correctly in this testcase.  If the value of 'a' is large 
enough to cause the multiplication to overflow, then your program has 
undefined behavior.

>
> Here is the reduced testcase:
>
> long long foo(int *a)
>
> {
>
> long long c;
>
> c = *a * 1405;
>
> return c;
>
> }
>
> Clang emitted the following IR (It is done by EmitMUL() in
> CGExprScalar.cpp, while
> CGF.getLangOpts().getSignedOverflowBehavior()=LangOptions::SOB_Undefined
> and CGF.SanOpts.has(SanitizerKind::SignedIntegerOverflow)=false):
>
> ; Function Attrs: nounwind readonly
>
> define i64 @foo(i32* nocapture readonly %a) #0 {
>
> entry:
>
>    %0 = load i32* %a, align 4, !tbaa !1
>
>    %mul = mul nsw i32 %0, 1405
>
>    %conv = sext i32 %mul to i64
>
>    ret i64 %conv
>
> }
>
> Question 1: Is it reasonable to say “mul” is “No Signed Wrap” ?  *a
> *1405 could overflow though.

The 'nsw' here means: "if the multiply overflows, the result is 
'poison'", i.e. overflow leads to undefined behavior.

http://llvm.org/docs/LangRef.html#poisonvalues

>
> Question 2: Why SignedOverflowBehavior is set to
> LangOptions::SOB_Undefined by default?

That's what the standard mandates.

>
> Question 3: What about CGF.SanOpts.has(SanitizerKind::SignedIntegerOverflow)

What about it?

>
> Because of nsw, and with Quentin patch (git-svn-id:
> https://llvm.org/svn/llvm-project/llvm/trunk@233753
> 91177308-0d34-0410-b5e6-96231b3b80d8), ExtLdPromotion() in
> CodeGenPrepare.cpp is able promote the sext to the following:
>
> define i64 @foo(i32* nocapture readonly %a) #0 {
>
> entry:
>
>    %0 = load i32* %a, align 4, !tbaa !1
>
>    %conv = sext i32 %0 to i64
>
>    %mul = mul nsw i64 %conv, 1405
>
>    ret i64 %mul
>
> }
>
> This promotion itself looks fine to me if nsw is true, and the final
> code becomes:
>
>          ldrsw    x8, [x0]
>
>          movz    w9, #0x57d
>
>          mul      x0, x8, x9
>
>          ret
>
> The results is different from a 32-bit mul then sext, at least for my
> testcase.
>
> Without nsw, ExtLdPromotion() didn’t change anything, and the result is
> correct.
>
> Any thoughts would be helpful.
>
> Regards
>
> Lawrence Hu
>
>
>
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-- 
Jon Roelofs
jonathan at codesourcery.com
CodeSourcery / Mentor Embedded