[LLVMdev] Compiler opt is turned off ?

[Somenath Chakraborty]

Hello,

I was trying to run few testcases and see how llvm optmizes different
scenarios. I have a small testcase like:


#include <stdio.h>



int a, b, c;



int

main()

{

    a = b + c;

    c = a;



    if (a == b)

        b = c;

    else

        b = a;



    printf( " a = %d \n ", a );

    return 0;

}



The corresponding llvm IR is ( clang test.c -S -emit-llvm -o - -O3 ) :



define i32 @main() nounwind uwtable {

entry:

  %0 = load i32* @b, align 4, !tbaa !4

  %1 = load i32* @c, align 4, !tbaa !4

  %add = add nsw i32 %1, %0

  store i32 %add, i32* @a, align 4, !tbaa !4

  store i32 %add, i32* @c, align 4, !tbaa !4

  store i32 %add, i32* @b, align 4, !tbaa !4

  %call = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([11
x i8]* @.str, i64 0, i64 0), i32 %add) nounwind

  ret i32 0

}

Here, I can see, although the *copy-propagation* is kicked in, but, *constant
propagation* is not happening. I assume the reason behind this is that a, b
and c are all global variables. Also, as no one is reading b and c
after the statement "a = b + c", should they not get optimized out ( from
"c = a" to end of function)?



Now, if a, b and c are made locals, all the optmizations are kicked in:



define i32 @main() nounwind uwtable {

entry:

  %call = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([11
x i8]* @.str, i64 0, i64 0), i32 0) nounwind

  ret i32 0

}

Are we little conservative on global variables even though *they don't have
readers* and debug mode is *OFF* ? Or, is it supported under some FLAG? Am
I missing something .... ?



Thanks.
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