[llvm] r290844 - [InstCombine] use combineMetadataForCSE instead of copying it; NFCI

[Davide Italiano via llvm-commits]

On Tue, Jan 3, 2017 at 7:58 AM, Sanjay Patel <spatel at rotateright.com> wrote:
> That's what the code comment would have us believe, but... :)
>
> define i8* @isa_impl_wrap(i8** %x) {
>   %t2 = alloca i8*
>   %t4 = load i8*, i8** %x, !nonnull !0
>   store i8* %t4, i8** %t2
>   %t6 = load i8*, i8** %t2
>   ret i8* %t6
> }
> !0 = !{}
>
> $ ./opt -instcombine cse_nonnull.ll -S
> ...
> define i8* @isa_impl_wrap(i8** %x) {
>   %t4 = load i8*, i8** %x, align 8, !nonnull !0 <--- nonnull preserved when
> earlier instruction has it
>   ret i8* %t4
> }
>

So, it's not that `combineMetadataForCSE` not behaving ideally. When
instcombine runs on your intermediate you don't call it at all.

Debug:
IC: Visiting:   %t6 = load i8*, i8** %t2
[...]

(gdb) p LI.dump()
  %t6 = load i8*, i8** %t2, align 8
$2 = void
(gdb) p IsLoadCSE
$3 = false
(gdb) list
853       if (Value *AvailableVal = FindAvailableLoadedValue(
854               &LI, LI.getParent(), BBI, DefMaxInstsToScan, AA,
&IsLoadCSE)) {
855         if (IsLoadCSE)
856           combineMetadataForCSE(cast<LoadInst>(AvailableVal), &LI);
857
858         return replaceInstUsesWith(
859             LI, Builder->CreateBitOrPointerCast(AvailableVal, LI.getType(),
860                                                 LI.getName() + ".cast"));
861       }

So we call RAUW without combining metadata, (unless I'm missing something).

-- 
Davide

"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare