[PATCH] D15777: [GlobalOpt] Globals used only in "main" can more easily be localized

[James Molloy via llvm-commits]

Hi,

I don't fully agree with this. It is only applicable in C or C++
non-freestanding modes. We shouldn't break other front ends or languages
that happen to use the symbol 'main'.

Chandler has suggested a 'entry point' or 'noreentry' attribute to be a
corollary to norecurse. Perhaps that might help here?

As is, I don't think we can do this.

James
On Fri, 25 Dec 2015 at 14:54, Vaivaswatha Nagaraj via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> vaivaswatha created this revision.
> vaivaswatha added reviewers: jmolloy, hfinkel.
> vaivaswatha added a subscriber: llvm-commits.
>
> This change relaxes the condition for a global, that is only used in
> "main", to be localized. This relaxation partially undoes the change done
> in http://reviews.llvm.org/rL253168.
>
> This is required because the condition that was added earlier, that a
> store to the global needs to dominate all other references to it in the
> function is not necessary if the only function that accesses the global is
> "main". So, similar to the condition prior to the above mentioned change,
> this change allows "main" as an exception.
>
> http://reviews.llvm.org/D15777
>
> Files:
>   lib/Transforms/IPO/GlobalOpt.cpp
>   test/Transforms/GlobalOpt/load-store-global.ll
>
> Index: test/Transforms/GlobalOpt/load-store-global.ll
> ===================================================================
> --- test/Transforms/GlobalOpt/load-store-global.ll
> +++ test/Transforms/GlobalOpt/load-store-global.ll
> @@ -36,3 +36,16 @@
>  ; CHECK-NOT: load
>  }
>
> +; A global used only in "main" can always be localized.
> + at b = internal global i64 13, align 8
> +; CHECK-NOT: @b
> +
> +define i64 @main(i32 %argc, i8** %argv) norecurse {
> +; CHECK: %b = alloca
> +; CHECK store i64 13
> +  %a = load i64, i64* @b
> +; CHECK %a = load i64
> +  store i64 27, i64* @b
> +; CHECK store i64 27
> +  ret i64 %a
> +}
> Index: lib/Transforms/IPO/GlobalOpt.cpp
> ===================================================================
> --- lib/Transforms/IPO/GlobalOpt.cpp
> +++ lib/Transforms/IPO/GlobalOpt.cpp
> @@ -1883,8 +1883,9 @@
>        GV->getType()->getAddressSpace() == 0 &&
>        !GV->isExternallyInitialized() &&
>        allNonInstructionUsersCanBeMadeInstructions(GV) &&
> -      GS.AccessingFunction->doesNotRecurse() &&
> -      isPointerValueDeadOnEntryToFunction(GS.AccessingFunction, GV) ) {
> +      ((GS.AccessingFunction->doesNotRecurse() &&
> +        isPointerValueDeadOnEntryToFunction(GS.AccessingFunction, GV)) ||
> +       (GS.AccessingFunction->getName() == "main"))) {
>      DEBUG(dbgs() << "LOCALIZING GLOBAL: " << *GV << "\n");
>      Instruction &FirstI = const_cast<Instruction&>(*GS.AccessingFunction
>
> ->getEntryBlock().begin());
>
>
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