[PATCH] D27560: Always run GlobalDCE at O0

[Duncan P. N. Exon Smith via llvm-commits]

Have you checked impact on -O0 compile time?

> On 2016-Dec-07, at 18:22, Mehdi AMINI via Phabricator <reviews at reviews.llvm.org> wrote:
> 
> mehdi_amini created this revision.
> mehdi_amini added a reviewer: chandlerc.
> mehdi_amini added subscribers: llvm-commits, dexonsmith.
> 
> Clang can emit unreachable call-graph from valid C++ code. There
> is no point keeping this into the binary, and also sometimes the
> generated code is broken and does not link, for example:
> 
> #include <iostream>
> #include <string>
> namespace {
> 
>  	struct VA {
>  	};
>  	struct A : virtual public VA {
>  		A() {
>  			static bool b = false;
>  			std::string str;
>  		}
>  	};
> 
> }
> int main(int argc, char* argv[]) {
> 
>  	A::A a;
> 
> }
> 
> See: http://lists.llvm.org/pipermail/cfe-dev/2016-October/051152.html
> 
> 
> https://reviews.llvm.org/D27560
> 
> Files:
>  llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
>  llvm/test/Other/GlobalDCE_at_O0.ll
> 
> 
> Index: llvm/test/Other/GlobalDCE_at_O0.ll
> ===================================================================
> --- /dev/null
> +++ llvm/test/Other/GlobalDCE_at_O0.ll
> @@ -0,0 +1,7 @@
> +; RUN: opt < %s -O0 -S | FileCheck %s
> +
> +; CHECK-NOT: @test1
> +
> +define internal i32 @test1() {
> +	ret i32 42
> +}
> Index: llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
> ===================================================================
> --- llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
> +++ llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
> @@ -405,6 +405,9 @@
>     else if (!GlobalExtensions->empty() || !Extensions.empty())
>       MPM.add(createBarrierNoopPass());
> 
> +    // Clang may generate unreachable call-graphs, let's prune it.
> +    MPM.add(createGlobalDCEPass());
> +
>     if (PrepareForThinLTO)
>       // Rename anon globals to be able to export them in the summary.
>       MPM.add(createNameAnonGlobalPass());
> 
> 
> <D27560.80706.patch>