FIX: Handle non-VLA compilers

Leverages the C std version and the standard __STDC_NO_VLA__ macro
to detect whether VLA is supported, and if not falls back to using alloca.

Original merge request: xiph/rnnoise#219

Author: marler8997

src/arch.h

@@ -192,4 +192,15 @@ static RENAMENOISE_INLINE int renamenoise_isnan(float x)
 #define RENAMENOISE_GLOBAL_STACK_SIZE 120000
 #endif
 
+#if __STDC_VERSION__ < 199901L || (__STDC_VERSION__ > 201000L && __STDC_NO_VLA__ == 1)
+   #define RENAMENOISE_NO_VLA
+#endif
+
+#ifdef RENAMENOISE_NO_VLA
+    #include <malloc.h>
+    #define renamenoise_stackalloc(type, id, len) type *id = alloca((len) * sizeof(type))
+#else
+    #define renamenoise_stackalloc(type, id, len) type id[len]
+#endif
+
 #endif /* ARCH_H */

src/pitch.c

@@ -235,9 +235,9 @@ void renamenoise_pitch_search(const renamenoise_val16 *x_lp, renamenoise_val16 *
    renamenoise_assert(max_pitch>0);
    lag = len+max_pitch;
 
-   renamenoise_val16 x_lp4[len>>2];
-   renamenoise_val16 y_lp4[lag>>2];
-   renamenoise_val32 xcorr[max_pitch>>1];
+   renamenoise_stackalloc(renamenoise_val16, x_lp4, len>>2);
+   renamenoise_stackalloc(renamenoise_val16, y_lp4, lag>>2);
+   renamenoise_stackalloc(renamenoise_val32, xcorr, max_pitch>>1);
 
    /* Downsample by 2 again */
    for (j=0;j<len>>2;j++)
@@ -313,7 +313,7 @@ renamenoise_val16 renamenoise_remove_doubling(renamenoise_val16 *x, int maxperio
       *T0_=maxperiod-1;
 
    T = T0 = *T0_;
-   renamenoise_val32 yy_lookup[maxperiod+1];
+   renamenoise_stackalloc(renamenoise_val32, yy_lookup, maxperiod+1);
    renamenoise_dual_inner_prod(x, x, x-T0, N, &xx, &xy);
    yy_lookup[0] = xx;
    yy=xx;

src/renamenoise_lpc.c

@@ -83,7 +83,7 @@ void renamenoise_fir(
          int ord)
 {
    int i,j;
-   renamenoise_val16 rnum[ord];
+   renamenoise_stackalloc(renamenoise_val16, rnum, ord);
    for(i=0;i<ord;i++)
       rnum[i] = num[ord-i-1];
    for (i=0;i<N-3;i+=4)
@@ -134,8 +134,8 @@ void renamenoise_iir(const renamenoise_val32 *_x,
 #else
    int i,j;
    renamenoise_assert((ord&3)==0);
-   renamenoise_val16 rden[ord];
-   renamenoise_val16 y[N+ord];
+   renamenoise_stackalloc(renamenoise_val16, rden, ord);
+   renamenoise_stackalloc(renamenoise_val16, y, N+ord);
    for(i=0;i<ord;i++)
       rden[i] = den[ord-i-1];
    for(i=0;i<ord;i++)
@@ -195,7 +195,7 @@ int _renamenoise_autocorr(
    int fastN=n-lag;
    int shift;
    const renamenoise_val16 *xptr;
-   renamenoise_val16 xx[n];
+   renamenoise_stackalloc(renamenoise_val16, xx, n);
    renamenoise_assert(n>0);
    renamenoise_assert(overlap>=0);
    if (overlap == 0)