Perfectly fine division throws floating point exception
1
I have a mystery floating point exception.
I catch it by doing:
feenableexcept( FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW );
Then the second division (last line) in this code:
const float dx = other.px[ j ] - curx;
const float dy = other.py[ j ] - cury;
const float dsqr = dx*dx + dy*dy;
ASSERTM( dsqr > 0.0f, "dx %f dy %f dsqr %f", dx, dy, dsqr );
const float dist = sqrtf( dsqr );
ASSERTM( dist > 0.0f, "dx %f dy %f dsqr %f dist %f", dx, dy, dsqr, dist );
LOGI( "dx %f dy %f dsqr %f dist %f", dx, dy, dsqr, dist );
const float dirx = dx / dist;
const float diry = dy / dist;
Throws a SIGFPE as follows:
The divisor (printed out in console, and also printed by gdb) is 1.0839119 and numerator is -1.05979919 so I don't see what is wrong with that.
For extra weirdness:
- Does not happen in -O0 debug build.
- Does not happen if I run the app through valgrind
Compiler:
$ clang -v
clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
And compiler options:
clang++ -DXWIN -DLANDSCAPE -DUSECOREPROFILE -Dlinux -D_POSIX_C_SOURCE=199309L -D_DEFAULT_SOURCE -DAPPVER=1.00 -DLOGTAG=minimal -I/home/bram/src/stb/ -I/home/bram/include -I/public -I/home/bram/src/dutch-blunt/src -I/home/bram/apps/GBase/src -IPI -I/home/bram/src/ThreadTracer `/home/bram/bin/sdl2-config --cflags` -g -Wall -Wshadow -Wno-conversion -Wno-missing-braces -Wno-old-style-cast -Wno-unknown-pragmas -MMD -MP -O2 -std=c++11 -c -o PI/stars.o PI/stars.cpp
Why is this FPE happening? The only reason I could think of, is that even though it is a scalar division, clang generates a SIMD division. What if the other lanes in the XMM register contain zero denominators? Do SIMD divisions generate FPEs if any of the lanes divides by zero? If so, how can I ever trap FPEs?
floating-point clang floating-point-exceptions
asked Dec 30 '18 at 23:56
BramBram
2,6562250
add a comment |
1
I have a mystery floating point exception.
I catch it by doing:
feenableexcept( FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW );
Then the second division (last line) in this code:
const float dx = other.px[ j ] - curx;
const float dy = other.py[ j ] - cury;
const float dsqr = dx*dx + dy*dy;
ASSERTM( dsqr > 0.0f, "dx %f dy %f dsqr %f", dx, dy, dsqr );
const float dist = sqrtf( dsqr );
ASSERTM( dist > 0.0f, "dx %f dy %f dsqr %f dist %f", dx, dy, dsqr, dist );
LOGI( "dx %f dy %f dsqr %f dist %f", dx, dy, dsqr, dist );
const float dirx = dx / dist;
const float diry = dy / dist;
Throws a SIGFPE as follows:
The divisor (printed out in console, and also printed by gdb) is 1.0839119 and numerator is -1.05979919 so I don't see what is wrong with that.
For extra weirdness:
- Does not happen in -O0 debug build.
- Does not happen if I run the app through valgrind
Compiler:
$ clang -v
clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
And compiler options:
clang++ -DXWIN -DLANDSCAPE -DUSECOREPROFILE -Dlinux -D_POSIX_C_SOURCE=199309L -D_DEFAULT_SOURCE -DAPPVER=1.00 -DLOGTAG=minimal -I/home/bram/src/stb/ -I/home/bram/include -I/public -I/home/bram/src/dutch-blunt/src -I/home/bram/apps/GBase/src -IPI -I/home/bram/src/ThreadTracer `/home/bram/bin/sdl2-config --cflags` -g -Wall -Wshadow -Wno-conversion -Wno-missing-braces -Wno-old-style-cast -Wno-unknown-pragmas -MMD -MP -O2 -std=c++11 -c -o PI/stars.o PI/stars.cpp
Why is this FPE happening? The only reason I could think of, is that even though it is a scalar division, clang generates a SIMD division. What if the other lanes in the XMM register contain zero denominators? Do SIMD divisions generate FPEs if any of the lanes divides by zero? If so, how can I ever trap FPEs?
floating-point clang floating-point-exceptions
asked Dec 30 '18 at 23:56
BramBram
2,6562250
Bram, I would say you were correct, the divps doc saysPerforms a SIMD divide of the four, eight or sixteen packed single-precision floating-point values in the first source operand (the second operand) by the four, eight or sixteen packed single-precision floating-point values in the second source operand (the third operand).This may be a compiler bug.
– paxdiablo
Dec 31 '18 at 0:05
1
I would think that, if it were going to use simd for two divisions, it would ensure the others (forced ones) would be valid. Maybe they never tested it with exceptions enabled. I'd suggest raising this on the clang bug site and see what they say.
– paxdiablo
Dec 31 '18 at 0:08
1
gccandclangbasically don't support non-default floating-point environments. You can try-fsanitize=float-divide-by-zeroinstead.
– Tavian Barnes
Dec 31 '18 at 0:10
add a comment |
1
1
1
I have a mystery floating point exception.
I catch it by doing:
feenableexcept( FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW );
Then the second division (last line) in this code:
const float dx = other.px[ j ] - curx;
const float dy = other.py[ j ] - cury;
const float dsqr = dx*dx + dy*dy;
ASSERTM( dsqr > 0.0f, "dx %f dy %f dsqr %f", dx, dy, dsqr );
const float dist = sqrtf( dsqr );
ASSERTM( dist > 0.0f, "dx %f dy %f dsqr %f dist %f", dx, dy, dsqr, dist );
LOGI( "dx %f dy %f dsqr %f dist %f", dx, dy, dsqr, dist );
const float dirx = dx / dist;
const float diry = dy / dist;
Throws a SIGFPE as follows:
The divisor (printed out in console, and also printed by gdb) is 1.0839119 and numerator is -1.05979919 so I don't see what is wrong with that.
For extra weirdness:
- Does not happen in -O0 debug build.
- Does not happen if I run the app through valgrind
Compiler:
$ clang -v
clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
And compiler options:
clang++ -DXWIN -DLANDSCAPE -DUSECOREPROFILE -Dlinux -D_POSIX_C_SOURCE=199309L -D_DEFAULT_SOURCE -DAPPVER=1.00 -DLOGTAG=minimal -I/home/bram/src/stb/ -I/home/bram/include -I/public -I/home/bram/src/dutch-blunt/src -I/home/bram/apps/GBase/src -IPI -I/home/bram/src/ThreadTracer `/home/bram/bin/sdl2-config --cflags` -g -Wall -Wshadow -Wno-conversion -Wno-missing-braces -Wno-old-style-cast -Wno-unknown-pragmas -MMD -MP -O2 -std=c++11 -c -o PI/stars.o PI/stars.cpp
Why is this FPE happening? The only reason I could think of, is that even though it is a scalar division, clang generates a SIMD division. What if the other lanes in the XMM register contain zero denominators? Do SIMD divisions generate FPEs if any of the lanes divides by zero? If so, how can I ever trap FPEs?
floating-point clang floating-point-exceptions
asked Dec 30 '18 at 23:56
BramBram
2,6562250
I have a mystery floating point exception.
I catch it by doing:
feenableexcept( FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW );
Then the second division (last line) in this code:
const float dx = other.px[ j ] - curx;
const float dy = other.py[ j ] - cury;
const float dsqr = dx*dx + dy*dy;
ASSERTM( dsqr > 0.0f, "dx %f dy %f dsqr %f", dx, dy, dsqr );
const float dist = sqrtf( dsqr );
ASSERTM( dist > 0.0f, "dx %f dy %f dsqr %f dist %f", dx, dy, dsqr, dist );
LOGI( "dx %f dy %f dsqr %f dist %f", dx, dy, dsqr, dist );
const float dirx = dx / dist;
const float diry = dy / dist;
Throws a SIGFPE as follows:
The divisor (printed out in console, and also printed by gdb) is 1.0839119 and numerator is -1.05979919 so I don't see what is wrong with that.
For extra weirdness:
- Does not happen in -O0 debug build.
- Does not happen if I run the app through valgrind
Compiler:
$ clang -v
clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
And compiler options:
clang++ -DXWIN -DLANDSCAPE -DUSECOREPROFILE -Dlinux -D_POSIX_C_SOURCE=199309L -D_DEFAULT_SOURCE -DAPPVER=1.00 -DLOGTAG=minimal -I/home/bram/src/stb/ -I/home/bram/include -I/public -I/home/bram/src/dutch-blunt/src -I/home/bram/apps/GBase/src -IPI -I/home/bram/src/ThreadTracer `/home/bram/bin/sdl2-config --cflags` -g -Wall -Wshadow -Wno-conversion -Wno-missing-braces -Wno-old-style-cast -Wno-unknown-pragmas -MMD -MP -O2 -std=c++11 -c -o PI/stars.o PI/stars.cpp
Why is this FPE happening? The only reason I could think of, is that even though it is a scalar division, clang generates a SIMD division. What if the other lanes in the XMM register contain zero denominators? Do SIMD divisions generate FPEs if any of the lanes divides by zero? If so, how can I ever trap FPEs?
floating-point clang floating-point-exceptions
floating-point clang floating-point-exceptions
asked Dec 30 '18 at 23:56
BramBram
2,6562250
asked Dec 30 '18 at 23:56
BramBram
2,6562250
asked Dec 30 '18 at 23:56
BramBram
2,6562250
asked Dec 30 '18 at 23:56
BramBram
2,6562250
asked Dec 30 '18 at 23:56
BramBram
2,6562250
2,6562250
Bram, I would say you were correct, the divps doc saysPerforms a SIMD divide of the four, eight or sixteen packed single-precision floating-point values in the first source operand (the second operand) by the four, eight or sixteen packed single-precision floating-point values in the second source operand (the third operand).This may be a compiler bug.
– paxdiablo
Dec 31 '18 at 0:05
1
I would think that, if it were going to use simd for two divisions, it would ensure the others (forced ones) would be valid. Maybe they never tested it with exceptions enabled. I'd suggest raising this on the clang bug site and see what they say.
– paxdiablo
Dec 31 '18 at 0:08
1
gccandclangbasically don't support non-default floating-point environments. You can try-fsanitize=float-divide-by-zeroinstead.
– Tavian Barnes
Dec 31 '18 at 0:10
add a comment |
Bram, I would say you were correct, the divps doc saysPerforms a SIMD divide of the four, eight or sixteen packed single-precision floating-point values in the first source operand (the second operand) by the four, eight or sixteen packed single-precision floating-point values in the second source operand (the third operand).This may be a compiler bug.
– paxdiablo
Dec 31 '18 at 0:05
1
I would think that, if it were going to use simd for two divisions, it would ensure the others (forced ones) would be valid. Maybe they never tested it with exceptions enabled. I'd suggest raising this on the clang bug site and see what they say.
– paxdiablo
Dec 31 '18 at 0:08
1
gccandclangbasically don't support non-default floating-point environments. You can try-fsanitize=float-divide-by-zeroinstead.
– Tavian Barnes
Dec 31 '18 at 0:10
Bram, I would say you were correct, the divps doc says
Performs a SIMD divide of the four, eight or sixteen packed single-precision floating-point values in the first source operand (the second operand) by the four, eight or sixteen packed single-precision floating-point values in the second source operand (the third operand). This may be a compiler bug.– paxdiablo
Dec 31 '18 at 0:05
Bram, I would say you were correct, the divps doc says
Performs a SIMD divide of the four, eight or sixteen packed single-precision floating-point values in the first source operand (the second operand) by the four, eight or sixteen packed single-precision floating-point values in the second source operand (the third operand). This may be a compiler bug.– paxdiablo
Dec 31 '18 at 0:05
1
1
I would think that, if it were going to use simd for two divisions, it would ensure the others (forced ones) would be valid. Maybe they never tested it with exceptions enabled. I'd suggest raising this on the clang bug site and see what they say.
– paxdiablo
Dec 31 '18 at 0:08
I would think that, if it were going to use simd for two divisions, it would ensure the others (forced ones) would be valid. Maybe they never tested it with exceptions enabled. I'd suggest raising this on the clang bug site and see what they say.
– paxdiablo
Dec 31 '18 at 0:08
1
1
gcc and clang basically don't support non-default floating-point environments. You can try -fsanitize=float-divide-by-zero instead.– Tavian Barnes
Dec 31 '18 at 0:10
gcc and clang basically don't support non-default floating-point environments. You can try -fsanitize=float-divide-by-zero instead.– Tavian Barnes
Dec 31 '18 at 0:10
add a comment |
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Bram, I would say you were correct, the divps doc says
Performs a SIMD divide of the four, eight or sixteen packed single-precision floating-point values in the first source operand (the second operand) by the four, eight or sixteen packed single-precision floating-point values in the second source operand (the third operand).This may be a compiler bug.– paxdiablo
Dec 31 '18 at 0:05
1
I would think that, if it were going to use simd for two divisions, it would ensure the others (forced ones) would be valid. Maybe they never tested it with exceptions enabled. I'd suggest raising this on the clang bug site and see what they say.
– paxdiablo
Dec 31 '18 at 0:08
1
gccandclangbasically don't support non-default floating-point environments. You can try-fsanitize=float-divide-by-zeroinstead.– Tavian Barnes
Dec 31 '18 at 0:10