[LLVMbugs] [Bug 2969] New: clang turns const arrays inside functions into allocas/ stores instead of "internal constant" arrays

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=2969

           Summary: clang turns const arrays inside functions into
                    allocas/stores instead of "internal constant" arrays
           Product: clang
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: LLVM Codegen
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: richard at godbee.net
                CC: llvmbugs at cs.uiuc.edu


Created an attachment (id=2144)
 --> (http://llvm.org/bugs/attachment.cgi?id=2144)
C source code + complete LLVM assembly output from llvm-gcc and clang

int f1(int n) {
        const int array_one[] = { 1, 2, 3 };
        return array_one[n];
}

int f2(int n) {
        static const int array_two[] = { 7, 8, 9 };
        return array_two[n];
}

/* Note that f1() and f2() are identical, except f1()'s int[] is "const" and
f2()'s int[] is "static const". */

**********

llvm-gcc (as shipped with Xcode 3.1.1) will emit "internal constant" arrays for
both array_one and array_two...

@array_one.1538 = internal constant [3 x i32] [ i32 1, i32 2, i32 3 ]          
; <[3 x i32]*> [#uses=1]
@array_two.1545 = internal constant [3 x i32] [ i32 7, i32 8, i32 9 ]          
; <[3 x i32]*> [#uses=1]

Like llvm-gcc, clang (trunk r58037) will emit an "internal constant" array for
array_two...

@f2.array_two = internal global [3 x i32] [ i32 7, i32 8, i32 9 ]              
; <[3 x i32]*> [#uses=1]

However, for array_one, clang will alloca space for the array and store each
element one-by-one inside of f1()...

define i32 @f1(i32 %n) nounwind {
entry:
        %array_one = alloca [3 x i32], align 4          ; <[3 x i32]*>
[#uses=4]
        %.array = getelementptr [3 x i32]* %array_one, i32 0, i32 0            
; <i32*> [#uses=1]
        store i32 1, i32* %.array
        %.array1 = getelementptr [3 x i32]* %array_one, i32 0, i32 1           
; <i32*> [#uses=1]
        store i32 2, i32* %.array1
        %.array2 = getelementptr [3 x i32]* %array_one, i32 0, i32 2           
; <i32*> [#uses=1]
        store i32 3, i32* %.array2
        %arrayidx = getelementptr [3 x i32]* %array_one, i32 0, i32 %n         
; <i32*> [#uses=1]
        %tmp3 = load i32* %arrayidx             ; <i32> [#uses=1]
        ret i32 %tmp3
}

opt -std-compile-args doesn't transform the alloca + stores into an "internal
constant" array, either.


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