[LLVMdev] optimization assumes malloc return is non-null

[Sandro Magi]

On Thu, May 1, 2008 at 6:54 PM, Chris Lattner <sabre at nondot.org> wrote:
>
>  > I don't see how this could be true in general, without either
>  > knowledge of the malloc implementation, which would be fine, or
>  > presuming knowledge of the target, which would not be fine. If
>  > "malloc(sizeof(int))" were changed to "malloc(3245677423)", would it
>  > still be eliminated?
>
>  Would it cause your head to explode if you knew that llvm optimizes this:
>
>  static char* G;
>  void foo() {
>    G = malloc(sizeof(char));
>  }
>  char get() { return *G; }
>  void set(char x) { *G = x; }
>
>  into this (note the lack of malloc):
>
>  @G.body = internal global i8 undef              ; <i8*> [#uses=2]
>  define i8 @get() signext nounwind  {
>  entry:
>         %tmp2 = load i8* @G.body, align 1               ; <i8> [#uses=1]
>         ret i8 %tmp2
>  }
>  define void @set(i8 signext  %x) nounwind  {
>  entry:
>         store i8 %x, i8* @G.body, align 1
>         ret void
>  }
>  define void @foo() nounwind  {
>  entry:
>         ret void
>  }
>
>  ?
>
>  This is safe even without "whole program" information.  I love the as-if
>  rule ;-)

No, that seems perfectly reasonable (assuming I understand the
translation), but only because char is so small. This optimization
seems reasonable when the object is of word size or smaller, as this
may simply

Sandro