[llvm-dev] Legality of transformation

[Johannes Doerfert via llvm-dev]

On 4/4/20 12:23 PM, Mehdi AMINI via llvm-dev wrote:
> On Sat, Apr 4, 2020 at 9:33 AM Akash Banerjee via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Please consider the following C code:
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>> *    #define SZ 2048    int main(void) {      int A[SZ];      int B[SZ];
>>      int i, tmp;      for (i = 0; i < SZ; i++) {        tmp = A[i];
>> B[i] = tmp;      }      assert(A[SZ/2] == B[SZ/2]);    }*
>>
>> On running -O1 followed by -reg2mem I get the following IR:
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>> *define dso_local i32 @main() local_unnamed_addr #0 {entry:  %A = alloca
>> [2048 x i32], align 16  %B = alloca [2048 x i32], align 16  %"reg2mem
>> alloca point" = bitcast i32 0 to i32  %arrayidx3 = getelementptr inbounds
>> [2048 x i32], [2048 x i32]* %A, i64 0, i64 1024  %0 = load i32, i32*
>> %arrayidx3, align 16  %arrayidx4 = getelementptr inbounds [2048 x i32],
>> [2048 x i32]* %B, i64 0, i64 1024  %1 = load i32, i32* %arrayidx4, align
>> 16  %cmp5 = icmp eq i32 %0, %1  %conv = zext i1 %cmp5 to i32  %call = call
>> i32 (i32, ...) bitcast (i32 (...)* @assert to i32 (i32, ...)*)(i32 %conv)
>> #2  ret i32 0}*
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>> It is my understanding that in the original C code the assert would never
>> fail, however in the optimized IR the assert might fail.
>>
> Reading uninitialized memory is undefined behavior in C I believe, so even
> without talking about LLVM IR semantics your original program is incorrect
> as soon a you read from A.

I don't think reading and writing undef values is not undefined behavior 
(in IR).

Using undef values in branches (and some other ways) is.

The "problem" here is, as Roman noted, the fact that you cannot assume 
either of these hold:

   `undef != undef` or `undef == undef`.


Cheers,

   Johannes


> clang ub.c -fsanitize=memory && ./a.out
>
> *==10365==WARNING: MemorySanitizer: use-of-uninitialized-value*
>
>      #0 0x496ce7 in main (/tmp/a.out+0x496ce7)
>
>      #1 0x7f2e71f27bba in __libc_start_main
> (/lib/x86_64-linux-gnu/libc.so.6+0x26bba)
>
>      #2 0x41e299 in _start (/tmp/a.out+0x41e299)
>
>
> SUMMARY: MemorySanitizer: use-of-uninitialized-value (/tmp/a.out+0x496ce7)
> in main
>
> Exiting
>
>
>
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