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dotprod_mmx.c

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+/* 16-bit signed integer dot product
+ * MMX assisted version; also for SSE
+ *
+ * Copyright 2004 Phil Karn
+ * May be used under the terms of the GNU Lesser General Public License (LGPL)
+ */
+#include <stdlib.h>
+#include "fec.h"
+
+struct dotprod {
+  int len; /* Number of coefficients */
+
+  /* On a MMX or SSE machine, these hold 4 copies of the coefficients,
+   * preshifted by 0,1,2,3 words to meet all possible input data
+   * alignments (see Intel ap559 on MMX dot products).
+   */
+  signed short *coeffs[4];
+};
+long dotprod_mmx_assist(signed short *a,signed short *b,int cnt);
+
+/* Create and return a descriptor for use with the dot product function */
+void *initdp_mmx(signed short coeffs[],int len){
+  struct dotprod *dp;
+  int i,j;
+
+
+  if(len == 0)
+    return NULL;
+
+  dp = (struct dotprod *)calloc(1,sizeof(struct dotprod));
+  dp->len = len;
+
+  /* Make 4 copies of coefficients, one for each data alignment */
+  for(i=0;i<4;i++){
+    dp->coeffs[i] = (signed short *)calloc(1+(len+i-1)/4,
+					   4*sizeof(signed short));
+    for(j=0;j<len;j++)
+      dp->coeffs[i][j+i] = coeffs[j];
+  }
+  return (void *)dp;
+}
+
+
+/* Free a dot product descriptor created earlier */
+void freedp_mmx(void *p){
+  struct dotprod *dp = (struct dotprod *)p;
+  int i;
+
+  for(i=0;i<4;i++)
+    if(dp->coeffs[i] != NULL)
+      free(dp->coeffs[i]);
+  free(dp);
+}
+
+/* Compute a dot product given a descriptor and an input array
+ * The length is taken from the descriptor
+ */
+long dotprod_mmx(void *p,signed short a[]){
+  struct dotprod *dp = (struct dotprod *)p;
+  int al;
+  signed short *ar;
+      
+  /* Round input data address down to 8 byte boundary
+   * NB: depending on the alignment of a[], memory
+   * before a[] will be accessed. The contents don't matter since they'll
+   * be multiplied by zero coefficients. I can't conceive of any
+   * situation where this could cause a segfault since memory protection
+   * in the x86 machines is done on much larger boundaries
+   */
+  ar = (signed short *)((int)a & ~7);
+  
+  /* Choose one of 4 sets of pre-shifted coefficients. al is both the
+   * index into dp->coeffs[] and the number of 0 words padded onto
+   * that coefficients array for alignment purposes
+   */
+  al = a - ar;
+  
+  /* Call assembler routine to do the work, passing number of 4-word blocks */
+  return dotprod_mmx_assist(ar,dp->coeffs[al],(dp->len+al-1)/4+1);
+}
+