Add PSHR32_ovflw() and use it in IMDCT

Prevents integer overflow UB in the shift rounding when the signal blows
up on bad bitstreams (if it triggers, the signal was already unusable
anyway).

Author: jmvalin

celt/arch.h

@@ -330,6 +330,8 @@ static OPUS_INLINE int celt_isnan(float x)
 #define SUB32(a,b) ((a)-(b))
 #define ADD32_ovflw(a,b) ((a)+(b))
 #define SUB32_ovflw(a,b) ((a)-(b))
+#define PSHR32_ovflw(a,shift) (a)
+
 #define MULT16_16_16(a,b)     ((a)*(b))
 #define MULT16_16(a,b)     ((opus_val32)(a)*(opus_val32)(b))
 #define MAC16_16(c,a,b)     ((c)+(opus_val32)(a)*(opus_val32)(b))

celt/fixed_debug.h

@@ -69,6 +69,8 @@ extern opus_int64 celt_mips;
 /* Avoid MSVC warning C4146: unary minus operator applied to unsigned type */
 /** Negate 32-bit value, ignore any overflows */
 #define NEG32_ovflw(a) (celt_mips+=2,(opus_val32)(0-(opus_uint32)(a)))
+/** 32-bit arithmetic shift right with rounding-to-nearest, ignoring overflows */
+#define PSHR32_ovflw(a,shift) (SHR32(ADD32_ovflw(a, (EXTEND32(1)<<(shift)>>1)),shift))
 
 static OPUS_INLINE short NEG16(int x)
 {

celt/fixed_generic.h

@@ -147,6 +147,8 @@
 /* Avoid MSVC warning C4146: unary minus operator applied to unsigned type */
 /** Negate 32-bit value, ignore any overflows */
 #define NEG32_ovflw(a) ((opus_val32)(0-(opus_uint32)(a)))
+/** 32-bit arithmetic shift right with rounding-to-nearest, ignoring overflows */
+#define PSHR32_ovflw(a,shift) (SHR32(ADD32_ovflw(a, (EXTEND32(1)<<(shift)>>1)),shift))
 
 /** 16x16 multiplication where the result fits in 16 bits */
 #define MULT16_16_16(a,b)     ((((opus_val16)(a))*((opus_val16)(b))))

celt/mdct.c

@@ -330,8 +330,8 @@ void clt_mdct_backward_c(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_sca
          t0 = t[i];
          t1 = t[N4+i];
          /* We'd scale up by 2 here, but instead it's done when mixing the windows */
-         yr = PSHR32(ADD32_ovflw(S_MUL(re,t0), S_MUL(im,t1)), IMDCT_HEADROOM);
-         yi = PSHR32(SUB32_ovflw(S_MUL(re,t1), S_MUL(im,t0)), IMDCT_HEADROOM);
+         yr = PSHR32_ovflw(ADD32_ovflw(S_MUL(re,t0), S_MUL(im,t1)), IMDCT_HEADROOM);
+         yi = PSHR32_ovflw(SUB32_ovflw(S_MUL(re,t1), S_MUL(im,t0)), IMDCT_HEADROOM);
          /* We swap real and imag because we're using an FFT instead of an IFFT. */
          re = yp1[1];
          im = yp1[0];
@@ -341,8 +341,8 @@ void clt_mdct_backward_c(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_sca
          t0 = t[(N4-i-1)];
          t1 = t[(N2-i-1)];
          /* We'd scale up by 2 here, but instead it's done when mixing the windows */
-         yr = PSHR32(ADD32_ovflw(S_MUL(re,t0), S_MUL(im,t1)), IMDCT_HEADROOM);
-         yi = PSHR32(SUB32_ovflw(S_MUL(re,t1), S_MUL(im,t0)), IMDCT_HEADROOM);
+         yr = PSHR32_ovflw(ADD32_ovflw(S_MUL(re,t0), S_MUL(im,t1)), IMDCT_HEADROOM);
+         yi = PSHR32_ovflw(SUB32_ovflw(S_MUL(re,t1), S_MUL(im,t0)), IMDCT_HEADROOM);
          yp1[0] = yr;
          yp0[1] = yi;
          yp0 += 2;