sqrtf()抛出一个域错误，尽管我防止负数。

Question

我正在对一个3D向量进行规范化，而clang-9生成的代码在sqrtf()上抛出一个SIGFPE，尽管我在调用它之前做了一个测试。

注意，我运行时启用了FP异常。

                        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;
                        }

哪个clang-9创建此程序集：

                        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;

SIGFPE显示了这个调用堆栈：

#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

SIGFPE似乎在__kernel_standard()调用中，是由0除以0引起的。

如果我在__kernel_standard()中查找'126‘类型，那么我看到它是:sqrtf(否定的)，并且调用此代码：

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;

在调试器中...causing此调用堆栈：

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>
[+]

此问题发生在clang-9 -O3中，而clang-9 -O0参数则不发生。

我使用的完整命令行：

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

为什么clang要计算负数的平方呢？它是否试图进行投机执行，并将基于lensq > FLT_EPSILON测试的结果混合起来？这是有效的吗？

Answer 1

抛出一个域错误，即使我防止负数。

但是if (lensq > FLT_EPSILON)太晚了，因为早期的dx*dx + dy*dy + dz*dz导致了int溢出。“并且确实溢出，导致lensq为负值”--这是未定义的行为UB。

编译器可以利用sqrtf(lensq)总是可以工作的优点，因为它可以假定dx*dx + dy*dy + dz*dz >= 0和lensq >= 0.0f。

把UB赶走。

// const float lensq = dx*dx + dy*dy + dz*dz;
const float lensq = 1LL*dx*dx + 1LL*dy*dy + 1LL*dz*dz;