[LLVMbugs] [Bug 10047] New: LLVM doesn't optimize a constructor that memsets-to-0 on a zero-initialized global

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=10047

           Summary: LLVM doesn't optimize a constructor that memsets-to-0
                    on a zero-initialized global
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Core LLVM classes
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: schaub.johannes at googlemail.com
                CC: llvmbugs at cs.uiuc.edu


Following code:

struct A {
  A():a() { }
  int a[100];
};

A a;

$ clang++ -S -O3 -emit-llvm main1.cpp -o -

This output can be further optimized:

; ModuleID = 'main1.cpp'
target datalayout =
"e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32-n8:16:32"
target triple = "i386-pc-linux-gnu"

%0 = type { i32, void ()* }
%struct.A = type { [100 x i32] }

@a = global %struct.A zeroinitializer, align 4
@llvm.global_ctors = appending global [1 x %0] [%0 { i32 65535, void ()*
@_GLOBAL__I_a }]

declare void @llvm.memset.p0i8.i64(i8* nocapture, i8, i64, i32, i1) nounwind

define internal void @_GLOBAL__I_a() nounwind {
  tail call void @llvm.memset.p0i8.i64(i8* bitcast (%struct.A* @a to i8*), i8
0, i64 400, i32 4, i1 false) nounwind
  ret void
}

The memset setting "@a" to zero can be optimized away, because it is already
zero. I was told to make a PR for this by nicholas on IRC.

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