[llvm] r313157 - ThinLTO: Correctly follow aliasee references when dead stripping.

[Hans Wennborg via llvm-commits]

I've reverted this in r313222 due to http://crbug.com/765004

On Wed, Sep 13, 2017 at 10:09 AM, Peter Collingbourne via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: pcc
> Date: Wed Sep 13 10:09:20 2017
> New Revision: 313157
>
> URL: http://llvm.org/viewvc/llvm-project?rev=313157&view=rev
> Log:
> ThinLTO: Correctly follow aliasee references when dead stripping.
>
> We were previously handling aliases during dead stripping by adding
> the aliased global's "original name" GUID to the worklist. This will
> lead to incorrect behaviour if the global has local linkage because
> the original name GUID will not correspond to the global's GUID in
> the summary.
>
> Because an alias is just another name for the global that it
> references, there is no need to mark the referenced global as used,
> or to follow references from any other copies of the global. So all
> we need to do is to follow references from the aliasee's summary
> instead of the alias.
>
> Differential Revision: https://reviews.llvm.org/D37789
>
> Added:
>     llvm/trunk/test/LTO/Resolution/X86/Inputs/dead-strip-alias.ll
>     llvm/trunk/test/LTO/Resolution/X86/dead-strip-alias.ll
> Modified:
>     llvm/trunk/lib/Transforms/IPO/FunctionImport.cpp
>
> Modified: llvm/trunk/lib/Transforms/IPO/FunctionImport.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/IPO/FunctionImport.cpp?rev=313157&r1=313156&r2=313157&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/IPO/FunctionImport.cpp (original)
> +++ llvm/trunk/lib/Transforms/IPO/FunctionImport.cpp Wed Sep 13 10:09:20 2017
> @@ -488,17 +488,14 @@ void llvm::computeDeadSymbols(
>    while (!Worklist.empty()) {
>      auto VI = Worklist.pop_back_val();
>      for (auto &Summary : VI.getSummaryList()) {
> -      for (auto Ref : Summary->refs())
> +      GlobalValueSummary *Base = Summary.get();
> +      if (auto *AS = dyn_cast<AliasSummary>(Base))
> +        Base = &AS->getAliasee();
> +      for (auto Ref : Base->refs())
>          visit(Ref);
> -      if (auto *FS = dyn_cast<FunctionSummary>(Summary.get()))
> +      if (auto *FS = dyn_cast<FunctionSummary>(Base))
>          for (auto Call : FS->calls())
>            visit(Call.first);
> -      if (auto *AS = dyn_cast<AliasSummary>(Summary.get())) {
> -        auto AliaseeGUID = AS->getAliasee().getOriginalName();
> -        ValueInfo AliaseeVI = Index.getValueInfo(AliaseeGUID);
> -        if (AliaseeVI)
> -          visit(AliaseeVI);
> -      }
>      }
>    }
>    Index.setWithGlobalValueDeadStripping();
>
> Added: llvm/trunk/test/LTO/Resolution/X86/Inputs/dead-strip-alias.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/LTO/Resolution/X86/Inputs/dead-strip-alias.ll?rev=313157&view=auto
> ==============================================================================
> --- llvm/trunk/test/LTO/Resolution/X86/Inputs/dead-strip-alias.ll (added)
> +++ llvm/trunk/test/LTO/Resolution/X86/Inputs/dead-strip-alias.ll Wed Sep 13 10:09:20 2017
> @@ -0,0 +1,4 @@
> +target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
> +target triple = "x86_64-unknown-linux-gnu"
> +
> + at external = global i8 42
>
> Added: llvm/trunk/test/LTO/Resolution/X86/dead-strip-alias.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/LTO/Resolution/X86/dead-strip-alias.ll?rev=313157&view=auto
> ==============================================================================
> --- llvm/trunk/test/LTO/Resolution/X86/dead-strip-alias.ll (added)
> +++ llvm/trunk/test/LTO/Resolution/X86/dead-strip-alias.ll Wed Sep 13 10:09:20 2017
> @@ -0,0 +1,20 @@
> +; RUN: opt -module-summary -o %t %s
> +; RUN: opt -module-summary -o %t2 %S/Inputs/dead-strip-alias.ll
> +; RUN: llvm-lto2 run %t -r %t,main,px -r %t,alias,p -r %t,external, \
> +; RUN:               %t2 -r %t2,external,p \
> +; RUN: -save-temps -o %t3
> +; RUN: llvm-nm %t3.1 | FileCheck %s
> +
> +; CHECK: D external
> +
> +target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
> +target triple = "x86_64-unknown-linux-gnu"
> +
> + at alias = alias i8*, i8** @internal
> +
> + at internal = internal global i8* @external
> + at external = external global i8
> +
> +define i8** @main() {
> +  ret i8** @alias
> +}
>
>
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