[llvm-commits] [llvm-gcc-4.2] r98626 - /llvm-gcc-4.2/trunk/gcc/llvm-backend.cpp

[Chris Lattner]

Hi Duncan,

This is not safe, please revert it. 

I originally tried to do this for llvm-gcc 3 and was defeated by libstdc++.  It assumes (and it turns out the c++ standard allows this) that global variables are zero initialized before the constructor for a type is run on it.

-Chris

On Mar 16, 2010, at 4:35 AM, Duncan Sands wrote:

> Author: baldrick
> Date: Tue Mar 16 06:35:32 2010
> New Revision: 98626
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=98626&view=rev
> Log:
> If a global variable is initialized by a code sequence, then do not zero
> initialize it: using 'undef' makes it easier for the optimizers to sink
> initialization code into the initializer for the global.  It is not clear
> if this is always correct, since it relies on the code sequence not somehow
> making use of the global being zero initialized.  My limited experiments
> suggest that everything is fine.  This is partial progress towards getting
> better code out of the testcase in PR6551.
> 
> Modified:
>    llvm-gcc-4.2/trunk/gcc/llvm-backend.cpp
> 
> Modified: llvm-gcc-4.2/trunk/gcc/llvm-backend.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm-gcc-4.2/trunk/gcc/llvm-backend.cpp?rev=98626&r1=98625&r2=98626&view=diff
> ==============================================================================
> --- llvm-gcc-4.2/trunk/gcc/llvm-backend.cpp (original)
> +++ llvm-gcc-4.2/trunk/gcc/llvm-backend.cpp Tue Mar 16 06:35:32 2010
> @@ -1320,10 +1320,17 @@
>   // Convert the initializer over.
>   Constant *Init;
>   if (DECL_INITIAL(decl) == 0 || DECL_INITIAL(decl) == error_mark_node) {
> -    // This global should be zero initialized.  Reconvert the type in case the
> +    // This global does not have an explicit initializer.  This usually means
> +    // that it should be zero initialized.  Reconvert the type in case the
>     // forward def of the global and the real def differ in type (e.g. declared
>     // as 'int A[]', and defined as 'int A[100]').
> -    Init = Constant::getNullValue(ConvertType(TREE_TYPE(decl)));
> +    if (TYPE_NEEDS_CONSTRUCTING(TREE_TYPE(decl))) {
> +      // The global will be initialized by a code sequence - it does not need to
> +      // be zero initialized.
> +      Init = UndefValue::get(ConvertType(TREE_TYPE(decl)));
> +    } else {
> +      Init = Constant::getNullValue(ConvertType(TREE_TYPE(decl)));
> +    }
>   } else {
>     assert((TREE_CONSTANT(DECL_INITIAL(decl)) || 
>             TREE_CODE(DECL_INITIAL(decl)) == STRING_CST) &&
> 
> 
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