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<div class="moz-cite-prefix">On 4/4/20 12:23 PM, Mehdi AMINI via
llvm-dev wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CANF-O=Y43qyEy1WZA+m3x8CwXpiyfBxvRi74XADG87jiMEMXaA@mail.gmail.com">
<pre class="moz-quote-pre" wrap="">On Sat, Apr 4, 2020 at 9:33 AM Akash Banerjee via llvm-dev <
<a class="moz-txt-link-abbreviated" href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">Please consider the following C code:
* #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:
*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}*
It is my understanding that in the original C code the assert would never
fail, however in the optimized IR the assert might fail.
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<pre class="moz-quote-pre" wrap="">
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.</pre>
</blockquote>
<p>I don't think reading and writing undef values is not undefined
behavior (in IR).<br>
</p>
<p>Using undef values in branches (and some other ways) is.<br>
</p>
<p>The "problem" here is, as Roman noted, the fact that you cannot
assume either of these hold:</p>
<p> `undef != undef` or `undef == undef`.<br>
</p>
<p><br>
</p>
<p>Cheers,</p>
<p> Johannes<br>
</p>
<p><br>
</p>
<blockquote type="cite"
cite="mid:CANF-O=Y43qyEy1WZA+m3x8CwXpiyfBxvRi74XADG87jiMEMXaA@mail.gmail.com">
<pre class="moz-quote-pre" wrap="">
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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<pre class="moz-quote-pre" wrap="">_______________________________________________
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