[llvm-bugs] [Bug 46627] New: Invalid optimization with math library `fmax` and signed zeros

via llvm-bugs llvm-bugs at lists.llvm.org
Tue Jul 7 18:58:51 PDT 2020 

https://bugs.llvm.org/show_bug.cgi?id=46627

            Bug ID: 46627
           Summary: Invalid optimization with math library `fmax` and
                    signed zeros
           Product: clang
           Version: 10.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: mpeddie at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

Created attachment 23697
  --> https://bugs.llvm.org/attachment.cgi?id=23697&action=edit
Preprocessed source for the example program

My simple test program gives different results depending on whether I enable
optimizations with -O, in the presence of -fno-builtin-fmax.  The program is as
follows (preprocessed source attached):

    #include <math.h>
    #include <stdio.h>

    int main(int argc __attribute__((unused)), char **argv
__attribute__((unused))) {
      const double a = fmax(0.0, -0.0);
      const double b = 1.0 / a;
      const int c = isinf(b);
      printf("Is b (%a) infinite? isinf() says: %d\n", b, c);

      return 0;
    }

No matter whether fmax returns 0.0 or -0.0, I expect b to be infinite and
isinf(b) to return a nonzero value.  If I compile this program without
optimizations, running it displays

    Is b (-inf) infinite? isinf() says: -1

as expected.  If I enable optimizations with -O and disable compile-time
evaluation with -fno-builtin-fmax, running it displays

    Is b (-inf) infinite? isinf() says: 0

If I pass the arguments to fmax in the opposite order

    const double a = fmax(-0.0, 0.0);

then isinf() returns 1 as expected.  If I assign 0.0 or -0.0 to a some other
way than by calling fmax(), then isinf() returns nonzero as expected.  The
example program in the similarly-titled
https://bugs.llvm.org/show_bug.cgi?id=37776 works correctly independent of
optimization settings.

The attached file test.i is the preprocessed source file.  The test program
compiles without errors or warning and never triggers the undefined-behavior
sanitizer.  The compiler invocation and complete output follows:

clang-10 -v -save-temps -O -fno-builtin-isinf -fno-builtin-fmax -std=c11 -o
test -fstack-protector-all -fstrict-overflow -D_FORTIFY_SOURCE=2 -fPIC
-fsanitize=undefined -fsanitize=address -Wall -Wextra -Werror -Wimplicit
-Wshadow -Wswitch-default test.c  -lm 
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.c -mrelocation-model pic -pic-level 2 -mthread-model posix
-mframe-pointer=none -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 -D _FORTIFY_SOURCE=2 -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 -O2
-Wall -Wextra -Werror -Wimplicit -Wshadow -Wswitch-default -std=c11
-fdebug-compilation-dir /home/peddie -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,object-size,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,object-size,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
-stack-protector 3 -fno-builtin-isinf -fno-builtin-fmax -fgnuc-version=4.2.1
-fobjc-runtime=gcc -fdiagnostics-show-option -fcolor-diagnostics
-vectorize-loops -vectorize-slp -faddrsig -o test.i -x c test.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.c -mrelocation-model pic -pic-level 2
-mthread-model posix -mframe-pointer=none -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 -O2 -Wall -Wextra -Werror
-Wimplicit -Wshadow -Wswitch-default -std=c11 -fdebug-compilation-dir
/home/peddie -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,object-size,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,object-size,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
-stack-protector 3 -fno-builtin-isinf -fno-builtin-fmax -fgnuc-version=4.2.1
-fobjc-runtime=gcc -fdiagnostics-show-option -fcolor-diagnostics
-vectorize-loops -vectorize-slp -disable-llvm-passes -faddrsig -o test.bc -x
cpp-output test.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.c -mrelocation-model pic -pic-level 2 -mthread-model posix
-mframe-pointer=none -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 -O2 -Wall -Wextra -Werror -Wimplicit -Wshadow
-Wswitch-default -std=c11 -fdebug-compilation-dir /home/peddie -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,object-size,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,object-size,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
-stack-protector 3 -fno-builtin-isinf -fno-builtin-fmax -fgnuc-version=4.2.1
-fobjc-runtime=gcc -fdiagnostics-show-option -fcolor-diagnostics
-vectorize-loops -vectorize-slp -faddrsig -o test.s -x ir test.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.c -target-cpu x86-64 -dwarf-version=4 -mrelocation-model
pic -o test.o test.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.o -lm --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

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