I am normalizing a 3D vector, and the clang-9 generated code throws a SIGFPE on the sqrtf() even though I do a test before calling it.
Note that I run with FP exceptions enabled.
const float lensq = dx*dx + dy*dy + dz*dz;
float invlen = 1.0f;
if (lensq > FLT_EPSILON)
{
const float leng = sqrtf(lensq);
invlen = 1.0f / leng;
}
Which clang-9 creates this assembly for:
const float lensq = dx*dx + dy*dy + dz*dz;
a11: 44 0f bf f1 movswl %cx,%r14d
a15: 44 89 f0 mov %r14d,%eax
a18: 41 0f af c6 imul %r14d,%eax
a1c: 44 0f bf ee movswl %si,%r13d
a20: 44 89 e9 mov %r13d,%ecx
a23: 41 0f af cd imul %r13d,%ecx
a27: 01 c1 add %eax,%ecx
a29: 44 0f bf fa movswl %dx,%r15d
a2d: 44 89 f8 mov %r15d,%eax
a30: 41 0f af c7 imul %r15d,%eax
a34: c5 f8 28 c7 vmovaps %xmm7,%xmm0
if (lensq > FLT_EPSILON)
a38: 01 c8 add %ecx,%eax
a3a: 74 2d je a69 <surface_extract_cases+0x7f9>
const float lensq = dx*dx + dy*dy + dz*dz;
a3c: c5 a2 2a c0 vcvtsi2ss %eax,%xmm11,%xmm0
a40: c4 c1 78 2e c0 vucomiss %xmm8,%xmm0
a45: 72 09 jb a50 <surface_extract_cases+0x7e0>
a47: c5 fa 51 c0 vsqrtss %xmm0,%xmm0,%xmm0
a4b: eb 18 jmp a65 <surface_extract_cases+0x7f5>
a4d: 0f 1f 00 nopl (%rax)
a50: c5 f8 77 vzeroupper
a53: e8 00 00 00 00 callq a58 <surface_extract_cases+0x7e8>
a58: c4 41 39 ef c0 vpxor %xmm8,%xmm8,%xmm8
a5d: c5 fa 10 3d 00 00 00 vmovss 0x0(%rip),%xmm7 # a65 <surface_extract_cases+0x7f5>
a64: 00
invlen = 1.0f / leng;
The SIGFPE shows this callstack:
#0 __kernel_standard (x=-1952511232, y=-1952511232, type=126) at ../sysdeps/ieee754/k_standard.c:584
#1 0x00007ffff5ea18e1 in __kernel_standard_f (x=<optimized out>, y=<optimized out>, type=<optimized out>) at ../sysdeps/ieee754/k_standardf.c:32
#2 0x000000000042a458 in mc_process_case_instances (caseidx=<optimized out>, numcases=31, fielddensity=0x3c171110, fieldtype=0x3c232510, isoval=<optimized out>, outputv=0x9c0d194 <scratch_surface_v+96744708>, outputn=0x12c0d194 <scratch_surface_n+96744708>, outputm=0x17e8ae3c <scratch_surface_m+32248236>, cases=<optimized out>) at ../../src/osino/src/surface.c:535
#3 surface_extract_cases (fielddensity=0x3c171110, fieldtype=0x3c232510, cases=<optimized out>, isoval=43, gridoff=<optimized out>, xlo=1120, xhi=48, ylo=32, yhi=48, zlo=16, zhi=32, outputv=0x9c09c90 <scratch_surface_v+96731136>, outputn=0x12c09c90 <scratch_surface_n+96731136>, outputm=0x17e89c90 <scratch_surface_m+32243712>, maxtria=65536, threadnr=1) at ../../src/osino/src/surface.c:745
The SIGFPE seems to be inside the __kernel_standard() call, and caused by dividing 0 by 0.
If I look up type '126' in __kernel_standard() then I see it is: sqrtf(negative) and this code is invoked:
case 126:
/* sqrt(x<0) */
exc.type = DOMAIN;
exc.name = type < 100 ? "sqrt" : "sqrtf";
if (_LIB_VERSION == _SVID_)
exc.retval = zero;
else
exc.retval = zero/zero;
if (_LIB_VERSION == _POSIX_)
errno = EDOM;
else if (!matherr(&exc)) {
/* if (_LIB_VERSION == _SVID_) {
(void) WRITE2("sqrt: DOMAIN error\n", 19);
} */
errno = EDOM;
}
break;
...causing this callstack in the debugger:
Thread 8 "noisetuner" received signal SIGFPE, Arithmetic exception.
[Switching to Thread 0x7fffccbdd700 (LWP 5838)]
─── Assembly ─────────────────────────────────────────────────────────────────────────────────────────
0x00007ffff5ea038c __kernel_standard+8236 nopl 0x0(%rax)
0x00007ffff5ea0390 __kernel_standard+8240 pxor %xmm0,%xmm0
0x00007ffff5ea0394 __kernel_standard+8244 cmp $0x2,%eax
0x00007ffff5ea0397 __kernel_standard+8247 divsd %xmm0,%xmm0
0x00007ffff5ea039b __kernel_standard+8251 movsd %xmm0,0x30(%rsp)
0x00007ffff5ea03a1 __kernel_standard+8257 jne 0x7ffff5e9f13b <__kernel_standard+3547>
0x00007ffff5ea03a7 __kernel_standard+8263 mov 0x38bc0a(%rip),%rax # 0x7ffff622bfb8
─── Expressions ──────────────────────────────────────────────────────────────────────────────────────
─── History ──────────────────────────────────────────────────────────────────────────────────────────
─── Memory ───────────────────────────────────────────────────────────────────────────────────────────
─── Registers ────────────────────────────────────────────────────────────────────────────────────────
rax 0x0000000000000002 rbx 0x00007ffff622c148 rcx 0x0000000058e94a00
rdx 0x00007ffff5f5a3c7 rsi 0x0000000000008d90 rdi 0x000000000000007e
rbp 0x00007fffccbdca30 rsp 0x00007fffccbdc640 r8 0x0000000000000006
r9 0x0000000000000005 r10 0x0000000000000038 r11 0x00007ffff5ea1460
r12 0x0000000000000001 r13 0x00000000ffff8d90 r14 0x00000000ffff9db0
r15 0x00000000ffff8e10 rip 0x00007ffff5ea0397 eflags [ PF ZF IF RF ]
cs 0x00000033 ss 0x0000002b ds 0x00000000
es 0x00000000 fs 0x00000000 gs 0x00000000
─── Source ───────────────────────────────────────────────────────────────────────────────────────────
Cannot display "/build/glibc-2ORdQG/glibc-2.27/math/../sysdeps/ieee754/k_standard.c" ([Errno 2] No such file or directory: '/build/glibc-2ORdQG/glibc-2.27/math/../sysdeps/ieee754/k_standard.c')
─── Stack ────────────────────────────────────────────────────────────────────────────────────────────
[0] from 0x00007ffff5ea0397 in __kernel_standard+8247 at ../sysdeps/ieee754/k_standard.c:584
arg x = -1952511232
arg y = -1952511232
arg type = 126
[1] from 0x00007ffff5ea18e1 in __kernel_standard_f+17 at ../sysdeps/ieee754/k_standardf.c:32
arg x = <optimized out>
arg y = <optimized out>
arg type = <optimized out>
[+]
The issue occurs with clang-9 -O3, but does not occur with clang-9 -O0 argument.
The full command line I use:
clang-9 -D_GNU_SOURCE -DAPPVER=1.00 -DUSECOREPROFILE -DNOUSESTEAM -DXWIN -DLANDSCAPE -DBLKMAG=6 -USTORECHARS -USTOREFP16 -DSTORESHORTS -I/home/bram/src/stb/ -I../GBase/src -Isrc -I../../src/dutch-blunt/src -I../../src/osino/src -I../../src/osino/src/../externals/enoki/include -I/usr/local/cuda/include -I/home/bram/src/zstd/lib -I../../src/ThreadTracer -IModels.game/geom `/opt/ode-master/bin/ode-config --cflags` `/usr/bin/sdl2-config --cflags` -g -Wall -pedantic -Wno-missing-braces -mavx2 -mfma -mf16c -MMD -MP -O3 -DDEBUG -c -o ../../src/osino/src/surface.o ../../src/osino/src/surface.c
Why is clang computing the sqrt of a negative number? Is it trying to do speculative execution, and blend the results based on the lensq > FLT_EPSILON test? Is that even valid?
throws a domain error, even thoug I guard against negative numbers
But if (lensq > FLT_EPSILON) is too late as earlier dx*dx + dy*dy + dz*dz caused int overflow. "and indeed overflow, causing lensq to be negative" - which is undefined behavior UB.
Compiler can take advantage that sqrtf(lensq) can always work because it can assume dx*dx + dy*dy + dz*dz >= 0 and so lensq >= 0.0f.
Get rid of UB.
// const float lensq = dx*dx + dy*dy + dz*dz;
const float lensq = 1LL*dx*dx + 1LL*dy*dy + 1LL*dz*dz;