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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Invalid optimization around abs() and integer comparison"
href="https://bugs.llvm.org/show_bug.cgi?id=46449">46449</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Invalid optimization around abs() and integer comparison
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>10.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>mpeddie@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=23648" name="attach_23648" title="Preprocessed source for the example program">attachment 23648</a> <a href="attachment.cgi?id=23648&action=edit" title="Preprocessed source for the example program">[details]</a></span>
Preprocessed source for the example program
My simple test program gives different results depending on whether or not I
enable optimizations with -O. The program is as follows (preprocessed source
attached):
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <inttypes.h>
#include <stdbool.h>
int main(int argc __attribute__((unused)), char **argv
__attribute__((unused))) {
const int32_t input[] = {INT32_MIN};
const int32_t a = input[0];
const int32_t absa = abs(a);
printf("abs(a): %"PRIi32"\n", absa);
const bool result = 0 > absa;
printf("0 > %" PRIi32 ": %s\n", absa, result ? "true" : "false");
printf("0 > %" PRIi32 ": %s\n", INT32_MIN, 0 > INT32_MIN ? "true" :
"false");
return 0;
}
If I enable optimizations when compiling and run this program, it prints out
abs(a): -2147483648
0 > -2147483648: false
0 > -2147483648: true
If I compile without optimizations, it prints out
abs(a): -2147483648
0 > -2147483648: true
0 > -2147483648: true
I expect both comparisons to be equal to true. While the result of the call to
`abs()` is undefined for this input value and may be anything, I've stored the
result (whatever it may be) into a variable, and I expect the value there
(which I see is negative when I print it) to compare less than zero independent
of how it was calculated.
Passing `-fno-builtin-abs` along with `-O` results in both comparisons being
true again as expected. Replacing the array access to define `a` with
`INT32_MIN` directly results in both comparisons being true again as expected.
The compiler invocation and complete output follows.
clang-10 -v -save-temps -std=c11 -O -o test -fwrapv -Wall -Wextra -Werror
-fsanitize=undefined -fsanitize=address test-on-concrete-abs-result.c
clang version 10.0.0-4
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/5
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/6.5.0
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/9
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64
"/usr/lib/llvm-10/bin/clang" -cc1 -triple x86_64-pc-linux-gnu -E
-save-temps=cwd -disable-free -disable-llvm-verifier -discard-value-names
-main-file-name test-on-concrete-abs-result.c -mrelocation-model static
-mthread-model posix -mframe-pointer=all -fmath-errno -fno-rounding-math
-masm-verbose -mconstructor-aliases -munwind-tables -target-cpu x86-64
-dwarf-column-info -fno-split-dwarf-inlining -debugger-tuning=gdb -v
-resource-dir /usr/lib/llvm-10/lib/clang/10.0.0 -internal-isystem
/usr/local/include -internal-isystem /usr/lib/llvm-10/lib/clang/10.0.0/include
-internal-externc-isystem /usr/include/x86_64-linux-gnu
-internal-externc-isystem /include -internal-externc-isystem /usr/include -Wall
-Wextra -Werror -std=c11 -fdebug-compilation-dir /home/peddie/butfor
-ferror-limit 19 -fmessage-length 0
-fsanitize=address,alignment,array-bounds,bool,builtin,enum,float-cast-overflow,function,integer-divide-by-zero,nonnull-attribute,null,pointer-overflow,return,returns-nonnull-attribute,shift-base,shift-exponent,signed-integer-overflow,unreachable,vla-bound,vptr
-fsanitize-recover=alignment,array-bounds,bool,builtin,enum,float-cast-overflow,function,integer-divide-by-zero,nonnull-attribute,null,pointer-overflow,returns-nonnull-attribute,shift-base,shift-exponent,signed-integer-overflow,vla-bound,vptr
-fsanitize-system-blacklist=/usr/lib/llvm-10/lib/clang/10.0.0/share/asan_blacklist.txt
-fsanitize-address-use-after-scope -fno-assume-sane-operator-new -fwrapv
-fgnuc-version=4.2.1 -fobjc-runtime=gcc -fdiagnostics-show-option
-fcolor-diagnostics -faddrsig -o test-on-concrete-abs-result.i -x c
test-on-concrete-abs-result.c
clang -cc1 version 10.0.0 based upon LLVM 10.0.0 default target
x86_64-pc-linux-gnu
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/local/include
/usr/lib/llvm-10/lib/clang/10.0.0/include
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
"/usr/lib/llvm-10/bin/clang" -cc1 -triple x86_64-pc-linux-gnu -emit-llvm-bc
-emit-llvm-uselists -save-temps=cwd -disable-free -disable-llvm-verifier
-discard-value-names -main-file-name test-on-concrete-abs-result.c
-mrelocation-model static -mthread-model posix -mframe-pointer=all -fmath-errno
-fno-rounding-math -masm-verbose -mconstructor-aliases -munwind-tables
-target-cpu x86-64 -dwarf-column-info -fno-split-dwarf-inlining
-debugger-tuning=gdb -v -resource-dir /usr/lib/llvm-10/lib/clang/10.0.0 -Wall
-Wextra -Werror -std=c11 -fdebug-compilation-dir /home/peddie/butfor
-ferror-limit 19 -fmessage-length 0
-fsanitize=address,alignment,array-bounds,bool,builtin,enum,float-cast-overflow,function,integer-divide-by-zero,nonnull-attribute,null,pointer-overflow,return,returns-nonnull-attribute,shift-base,shift-exponent,signed-integer-overflow,unreachable,vla-bound,vptr
-fsanitize-recover=alignment,array-bounds,bool,builtin,enum,float-cast-overflow,function,integer-divide-by-zero,nonnull-attribute,null,pointer-overflow,returns-nonnull-attribute,shift-base,shift-exponent,signed-integer-overflow,vla-bound,vptr
-fsanitize-system-blacklist=/usr/lib/llvm-10/lib/clang/10.0.0/share/asan_blacklist.txt
-fsanitize-address-use-after-scope -fno-assume-sane-operator-new -fwrapv
-fgnuc-version=4.2.1 -fobjc-runtime=gcc -fdiagnostics-show-option
-fcolor-diagnostics -disable-llvm-passes -faddrsig -o
test-on-concrete-abs-result.bc -x cpp-output test-on-concrete-abs-result.i
clang -cc1 version 10.0.0 based upon LLVM 10.0.0 default target
x86_64-pc-linux-gnu
#include "..." search starts here:
End of search list.
"/usr/lib/llvm-10/bin/clang" -cc1 -triple x86_64-pc-linux-gnu -S
-save-temps=cwd -disable-free -disable-llvm-verifier -discard-value-names
-main-file-name test-on-concrete-abs-result.c -mrelocation-model static
-mthread-model posix -mframe-pointer=all -fmath-errno -fno-rounding-math
-masm-verbose -mconstructor-aliases -munwind-tables -target-cpu x86-64
-dwarf-column-info -fno-split-dwarf-inlining -debugger-tuning=gdb -v
-resource-dir /usr/lib/llvm-10/lib/clang/10.0.0 -Wall -Wextra -Werror -std=c11
-fdebug-compilation-dir /home/peddie/butfor -ferror-limit 19 -fmessage-length 0
-fsanitize=address,alignment,array-bounds,bool,builtin,enum,float-cast-overflow,function,integer-divide-by-zero,nonnull-attribute,null,pointer-overflow,return,returns-nonnull-attribute,shift-base,shift-exponent,signed-integer-overflow,unreachable,vla-bound,vptr
-fsanitize-recover=alignment,array-bounds,bool,builtin,enum,float-cast-overflow,function,integer-divide-by-zero,nonnull-attribute,null,pointer-overflow,returns-nonnull-attribute,shift-base,shift-exponent,signed-integer-overflow,vla-bound,vptr
-fsanitize-system-blacklist=/usr/lib/llvm-10/lib/clang/10.0.0/share/asan_blacklist.txt
-fsanitize-address-use-after-scope -fno-assume-sane-operator-new -fwrapv
-fgnuc-version=4.2.1 -fobjc-runtime=gcc -fdiagnostics-show-option
-fcolor-diagnostics -faddrsig -o test-on-concrete-abs-result.s -x ir
test-on-concrete-abs-result.bc
clang -cc1 version 10.0.0 based upon LLVM 10.0.0 default target
x86_64-pc-linux-gnu
"/usr/lib/llvm-10/bin/clang" -cc1as -triple x86_64-pc-linux-gnu -filetype obj
-main-file-name test-on-concrete-abs-result.c -target-cpu x86-64
-dwarf-version=4 -mrelocation-model static -mrelax-all -o
test-on-concrete-abs-result.o test-on-concrete-abs-result.s
"/usr/bin/ld" --hash-style=both --build-id --eh-frame-hdr -m elf_x86_64
-dynamic-linker /lib64/ld-linux-x86-64.so.2 -o test
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/crt1.o
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/crti.o
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/crtbegin.o
-L/usr/bin/../lib/gcc/x86_64-linux-gnu/10
-L/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu
-L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu
-L/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../..
-L/usr/lib/llvm-10/bin/../lib -L/lib -L/usr/lib --whole-archive
/usr/lib/llvm-10/lib/clang/10.0.0/lib/linux/libclang_rt.asan-x86_64.a
--no-whole-archive
--dynamic-list=/usr/lib/llvm-10/lib/clang/10.0.0/lib/linux/libclang_rt.asan-x86_64.a.syms
test-on-concrete-abs-result.o --no-as-needed -lpthread -lrt -lm -ldl -lgcc
--as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/crtend.o
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/crtn.o</pre>
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