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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimization: llvm unable to remove bounds check"
href="https://bugs.llvm.org/show_bug.cgi?id=48909">48909</a>
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<th>Summary</th>
<td>Missed optimization: llvm unable to remove bounds check
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>alex.gaynor@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre><a href="https://godbolt.org/z/vEfPYj">https://godbolt.org/z/vEfPYj</a> (also below)
LLVM should be able to remove the branch that leads to the call to abort() --
data.length is known to be the same as block_len, and block_len is known not to
be 0. Therefore llvm should know that block_len - is always < data.length.
Source:
#include <stdlib.h>
#include <stdbool.h>
#include <stdint.h>
typedef struct {
uint8_t *data;
size_t length;
} slice;
static inline uint8_t subscript(slice data, size_t index) {
if (index >= data.length) {
abort();
}
return data.data[index];
}
bool check_padding(slice data, size_t block_len) {
if (data.length != block_len || block_len == 0) {
return false;
}
uint8_t pad_size = subscript(data, block_len - 1);
return pad_size > 7;
}
generated assembly:
check_padding: # @check_padding
push rax
xor eax, eax
cmp rsi, rdx
jne .LBB0_4
test rdx, rdx
je .LBB0_4
test rsi, rsi
je .LBB0_5
cmp byte ptr [rsi + rdi - 1], 7
seta al
.LBB0_4:
pop rcx
ret
.LBB0_5:
call abort</pre>
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