gcc bad codegen with union

[Sopel97]

GCC fails to keep v2df_t variables in register and writes them back onto the stack often.
I got 40% performance improvement by using __m128 directly where possible (i7-920).
Clang doesn't have that problem.

before: https://godbolt.org/z/cNxgkG
after: https://godbolt.org/z/55MvWD
may require to change loaddup to something else to compile on godbolt

/* Create macros so that the matrices are stored in column-major order */

#define A(i,j) a[ (j)*lda + (i) ]
#define B(i,j) b[ (j)*ldb + (i) ]
#define C(i,j) c[ (j)*ldc + (i) ]

/* Block sizes */
#define mc 256
#define kc 128
#define nb 1000

#define min( i, j ) ( (i)<(j) ? (i): (j) )

/* Routine for computing C = A * B + C */

void AddDot4x4( int, double *, int, double *, int, double *, int );
void PackMatrixA( int, double *, int, double * );
void PackMatrixB( int, double *, int, double * );
void InnerKernel( int, int, int, double *, int, double *, int, double *, int, int );

void MY_MMult( int m, int n, int k, double *a, int lda,
                                    double *b, int ldb,
                                    double *c, int ldc )
{
  int i, p, pb, ib;

  /* This time, we compute a mc x n block of C by a call to the InnerKernel */

  for ( p=0; p<k; p+=kc ){
    pb = min( k-p, kc );
    for ( i=0; i<m; i+=mc ){
      ib = min( m-i, mc );
      InnerKernel( ib, n, pb, &A( i,p ), lda, &B(p, 0 ), ldb, &C( i,0 ), ldc, i==0 );
    }
  }
}

void InnerKernel( int m, int n, int k, double *a, int lda,
                                       double *b, int ldb,
                                       double *c, int ldc, int first_time )
{
  int i, j;
  double
    packedA[ m * k ];
  static double
    packedB[ kc*nb ];    /* Note: using a static buffer is not thread safe... */

  for ( j=0; j<n; j+=4 ){        /* Loop over the columns of C, unrolled by 4 */
    if ( first_time )
      PackMatrixB( k, &B( 0, j ), ldb, &packedB[ j*k ] );
    for ( i=0; i<m; i+=4 ){        /* Loop over the rows of C */
      /* Update C( i,j ), C( i,j+1 ), C( i,j+2 ), and C( i,j+3 ) in
   one routine (four inner products) */
      if ( j == 0 )
  PackMatrixA( k, &A( i, 0 ), lda, &packedA[ i*k ] );
      AddDot4x4( k, &packedA[ i*k ], 4, &packedB[ j*k ], k, &C( i,j ), ldc );
    }
  }
}

void PackMatrixA( int k, double *a, int lda, double *a_to )
{
  int j;

  for( j=0; j<k; j++){  /* loop over columns of A */
    double
      *a_ij_pntr = &A( 0, j );

    *a_to     = *a_ij_pntr;
    *(a_to+1) = *(a_ij_pntr+1);
    *(a_to+2) = *(a_ij_pntr+2);
    *(a_to+3) = *(a_ij_pntr+3);

    a_to += 4;
  }
}

void PackMatrixB( int k, double *b, int ldb, double *b_to )
{
  int i;
  double
    *b_i0_pntr = &B( 0, 0 ), *b_i1_pntr = &B( 0, 1 ),
    *b_i2_pntr = &B( 0, 2 ), *b_i3_pntr = &B( 0, 3 );

  for( i=0; i<k; i++){  /* loop over rows of B */
    *b_to++ = *b_i0_pntr++;
    *b_to++ = *b_i1_pntr++;
    *b_to++ = *b_i2_pntr++;
    *b_to++ = *b_i3_pntr++;
  }
}

#include <mmintrin.h>
#include <xmmintrin.h>  // SSE
#include <pmmintrin.h>  // SSE2
#include <emmintrin.h>  // SSE3

typedef union
{
  __m128d v;
  double d[2];
} v2df_t;

void AddDot4x4( int k, double *a, int lda,  double *b, int ldb, double *c, int ldc )
{
  /* So, this routine computes a 4x4 block of matrix A
           C( 0, 0 ), C( 0, 1 ), C( 0, 2 ), C( 0, 3 ).
           C( 1, 0 ), C( 1, 1 ), C( 1, 2 ), C( 1, 3 ).
           C( 2, 0 ), C( 2, 1 ), C( 2, 2 ), C( 2, 3 ).
           C( 3, 0 ), C( 3, 1 ), C( 3, 2 ), C( 3, 3 ).
     Notice that this routine is called with c = C( i, j ) in the
     previous routine, so these are actually the elements
           C( i  , j ), C( i  , j+1 ), C( i  , j+2 ), C( i  , j+3 )
           C( i+1, j ), C( i+1, j+1 ), C( i+1, j+2 ), C( i+1, j+3 )
           C( i+2, j ), C( i+2, j+1 ), C( i+2, j+2 ), C( i+2, j+3 )
           C( i+3, j ), C( i+3, j+1 ), C( i+3, j+2 ), C( i+3, j+3 )

     in the original matrix C
     And now we use vector registers and instructions */

  int p;
  __m128d
    c_00_c_10_vreg,    c_01_c_11_vreg,    c_02_c_12_vreg,    c_03_c_13_vreg,
    c_20_c_30_vreg,    c_21_c_31_vreg,    c_22_c_32_vreg,    c_23_c_33_vreg,
    a_0p_a_1p_vreg,
    a_2p_a_3p_vreg,
    b_p0_vreg, b_p1_vreg, b_p2_vreg, b_p3_vreg;

  c_00_c_10_vreg = _mm_setzero_pd();
  c_01_c_11_vreg = _mm_setzero_pd();
  c_02_c_12_vreg = _mm_setzero_pd();
  c_03_c_13_vreg = _mm_setzero_pd();
  c_20_c_30_vreg = _mm_setzero_pd();
  c_21_c_31_vreg = _mm_setzero_pd();
  c_22_c_32_vreg = _mm_setzero_pd();
  c_23_c_33_vreg = _mm_setzero_pd();

  for ( p=0; p<k; p++ ){
    a_0p_a_1p_vreg = _mm_load_pd( (double *) a );
    a_2p_a_3p_vreg = _mm_load_pd( (double *) ( a+2 ) );
    a += 4;

    b_p0_vreg = _mm_loaddup_pd( (double *) b );       /* load and duplicate */
    b_p1_vreg = _mm_loaddup_pd( (double *) (b+1) );   /* load and duplicate */
    b_p2_vreg = _mm_loaddup_pd( (double *) (b+2) );   /* load and duplicate */
    b_p3_vreg = _mm_loaddup_pd( (double *) (b+3) );   /* load and duplicate */

    b += 4;

    /* First row and second rows */
    c_00_c_10_vreg += a_0p_a_1p_vreg * b_p0_vreg;
    c_01_c_11_vreg += a_0p_a_1p_vreg * b_p1_vreg;
    c_02_c_12_vreg += a_0p_a_1p_vreg * b_p2_vreg;
    c_03_c_13_vreg += a_0p_a_1p_vreg * b_p3_vreg;

    /* Third and fourth rows */
    c_20_c_30_vreg += a_2p_a_3p_vreg * b_p0_vreg;
    c_21_c_31_vreg += a_2p_a_3p_vreg * b_p1_vreg;
    c_22_c_32_vreg += a_2p_a_3p_vreg * b_p2_vreg;
    c_23_c_33_vreg += a_2p_a_3p_vreg * b_p3_vreg;
  }
  v2df_t
    c_00_c_10_vregx,    c_01_c_11_vregx,    c_02_c_12_vregx,    c_03_c_13_vregx,
    c_20_c_30_vregx,    c_21_c_31_vregx,    c_22_c_32_vregx,    c_23_c_33_vregx;

  c_00_c_10_vregx.v = c_00_c_10_vreg;
  c_01_c_11_vregx.v = c_01_c_11_vreg;
  c_02_c_12_vregx.v = c_02_c_12_vreg;
  c_03_c_13_vregx.v = c_03_c_13_vreg;
  c_20_c_30_vregx.v = c_20_c_30_vreg;
  c_21_c_31_vregx.v = c_21_c_31_vreg;
  c_22_c_32_vregx.v = c_22_c_32_vreg;
  c_23_c_33_vregx.v = c_23_c_33_vreg;

  C( 0, 0 ) += c_00_c_10_vregx.d[0];  C( 0, 1 ) += c_01_c_11_vregx.d[0];
  C( 0, 2 ) += c_02_c_12_vregx.d[0];  C( 0, 3 ) += c_03_c_13_vregx.d[0];

  C( 1, 0 ) += c_00_c_10_vregx.d[1];  C( 1, 1 ) += c_01_c_11_vregx.d[1];
  C( 1, 2 ) += c_02_c_12_vregx.d[1];  C( 1, 3 ) += c_03_c_13_vregx.d[1];

  C( 2, 0 ) += c_20_c_30_vregx.d[0];  C( 2, 1 ) += c_21_c_31_vregx.d[0];
  C( 2, 2 ) += c_22_c_32_vregx.d[0];  C( 2, 3 ) += c_23_c_33_vregx.d[0];

  C( 3, 0 ) += c_20_c_30_vregx.d[1];  C( 3, 1 ) += c_21_c_31_vregx.d[1];
  C( 3, 2 ) += c_22_c_32_vregx.d[1];  C( 3, 3 ) += c_23_c_33_vregx.d[1];
}