The DPPS (Dot Product) instruction is defined to first sum pairs of intermediate results, then sum those values to get the final result. i.e. (A+B)+(C+D)
We incrementally sum the results, i.e. ((A+B)+C)+D, which can result in incorrect rouding. For consistency, also remove the redundant (but harmless) add operation from DPPD Signed-off-by: Paul Brook <p...@nowt.org> --- target/i386/ops_sse.h | 47 +++++++++++++++++++++++-------------------- 1 file changed, 25 insertions(+), 22 deletions(-) diff --git a/target/i386/ops_sse.h b/target/i386/ops_sse.h index 535440f882..a5a48a20f6 100644 --- a/target/i386/ops_sse.h +++ b/target/i386/ops_sse.h @@ -1934,32 +1934,36 @@ SSE_HELPER_I(helper_pblendw, W, 8, FBLENDP) void glue(helper_dpps, SUFFIX)(CPUX86State *env, Reg *d, Reg *s, uint32_t mask) { - float32 iresult = float32_zero; + float32 prod, iresult, iresult2; + /* + * We must evaluate (A+B)+(C+D), not ((A+B)+C)+D + * to correctly round the intermediate results + */ if (mask & (1 << 4)) { - iresult = float32_add(iresult, - float32_mul(d->ZMM_S(0), s->ZMM_S(0), - &env->sse_status), - &env->sse_status); + iresult = float32_mul(d->ZMM_S(0), s->ZMM_S(0), &env->sse_status); + } else { + iresult = float32_zero; } if (mask & (1 << 5)) { - iresult = float32_add(iresult, - float32_mul(d->ZMM_S(1), s->ZMM_S(1), - &env->sse_status), - &env->sse_status); + prod = float32_mul(d->ZMM_S(1), s->ZMM_S(1), &env->sse_status); + } else { + prod = float32_zero; } + iresult = float32_add(iresult, prod, &env->sse_status); if (mask & (1 << 6)) { - iresult = float32_add(iresult, - float32_mul(d->ZMM_S(2), s->ZMM_S(2), - &env->sse_status), - &env->sse_status); + iresult2 = float32_mul(d->ZMM_S(2), s->ZMM_S(2), &env->sse_status); + } else { + iresult2 = float32_zero; } if (mask & (1 << 7)) { - iresult = float32_add(iresult, - float32_mul(d->ZMM_S(3), s->ZMM_S(3), - &env->sse_status), - &env->sse_status); + prod = float32_mul(d->ZMM_S(3), s->ZMM_S(3), &env->sse_status); + } else { + prod = float32_zero; } + iresult2 = float32_add(iresult2, prod, &env->sse_status); + iresult = float32_add(iresult, iresult2, &env->sse_status); + d->ZMM_S(0) = (mask & (1 << 0)) ? iresult : float32_zero; d->ZMM_S(1) = (mask & (1 << 1)) ? iresult : float32_zero; d->ZMM_S(2) = (mask & (1 << 2)) ? iresult : float32_zero; @@ -1968,13 +1972,12 @@ void glue(helper_dpps, SUFFIX)(CPUX86State *env, Reg *d, Reg *s, uint32_t mask) void glue(helper_dppd, SUFFIX)(CPUX86State *env, Reg *d, Reg *s, uint32_t mask) { - float64 iresult = float64_zero; + float64 iresult; if (mask & (1 << 4)) { - iresult = float64_add(iresult, - float64_mul(d->ZMM_D(0), s->ZMM_D(0), - &env->sse_status), - &env->sse_status); + iresult = float64_mul(d->ZMM_D(0), s->ZMM_D(0), &env->sse_status); + } else { + iresult = float64_zero; } if (mask & (1 << 5)) { iresult = float64_add(iresult, -- 2.36.0