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[PATCH] i386: DPPS rounding fix
From: |
Paolo Bonzini |
Subject: |
[PATCH] i386: DPPS rounding fix |
Date: |
Thu, 25 Aug 2022 18:48:27 +0200 |
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 change the variable names to the ones used
in the Intel SDM and implement DPPD following the manual.
Based on a patch by Paul Brook <paul@nowt.org>.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
target/i386/ops_sse.h | 67 ++++++++++++++++++++++---------------------
1 file changed, 35 insertions(+), 32 deletions(-)
diff --git a/target/i386/ops_sse.h b/target/i386/ops_sse.h
index 2524db4c25..b12b271fcd 100644
--- a/target/i386/ops_sse.h
+++ b/target/i386/ops_sse.h
@@ -1943,56 +1943,59 @@ 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 prod1, prod2, temp2, temp3, temp4;
+ /*
+ * 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);
+ prod1 = float32_mul(d->ZMM_S(0), s->ZMM_S(0), &env->sse_status);
+ } else {
+ prod1 = 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);
+ prod2 = float32_mul(d->ZMM_S(1), s->ZMM_S(1), &env->sse_status);
+ } else {
+ prod2 = float32_zero;
}
+ temp2 = float32_add(prod1, prod2, &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);
+ prod1 = float32_mul(d->ZMM_S(2), s->ZMM_S(2), &env->sse_status);
+ } else {
+ prod1 = 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);
+ prod2 = float32_mul(d->ZMM_S(3), s->ZMM_S(3), &env->sse_status);
+ } else {
+ prod2 = float32_zero;
}
- 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;
- d->ZMM_S(3) = (mask & (1 << 3)) ? iresult : float32_zero;
+ temp3 = float32_add(prod1, prod2, &env->sse_status);
+ temp4 = float32_add(temp2, temp3, &env->sse_status);
+
+ d->ZMM_S(0) = (mask & (1 << 0)) ? temp4 : float32_zero;
+ d->ZMM_S(1) = (mask & (1 << 1)) ? temp4 : float32_zero;
+ d->ZMM_S(2) = (mask & (1 << 2)) ? temp4 : float32_zero;
+ d->ZMM_S(3) = (mask & (1 << 3)) ? temp4 : float32_zero;
}
void glue(helper_dppd, SUFFIX)(CPUX86State *env, Reg *d, Reg *s, uint32_t mask)
{
- float64 iresult = float64_zero;
+ float64 prod1, prod2, temp2;
if (mask & (1 << 4)) {
- iresult = float64_add(iresult,
- float64_mul(d->ZMM_D(0), s->ZMM_D(0),
- &env->sse_status),
- &env->sse_status);
+ prod1 = float64_mul(d->ZMM_D(0), s->ZMM_D(0), &env->sse_status);
+ } else {
+ prod1 = float64_zero;
}
if (mask & (1 << 5)) {
- iresult = float64_add(iresult,
- float64_mul(d->ZMM_D(1), s->ZMM_D(1),
- &env->sse_status),
- &env->sse_status);
+ prod2 = float64_mul(d->ZMM_D(1), s->ZMM_D(1), &env->sse_status);
+ } else {
+ prod2 = float64_zero;
}
- d->ZMM_D(0) = (mask & (1 << 0)) ? iresult : float64_zero;
- d->ZMM_D(1) = (mask & (1 << 1)) ? iresult : float64_zero;
+ temp2 = float64_add(prod1, prod2, &env->sse_status);
+ d->ZMM_D(0) = (mask & (1 << 0)) ? temp2 : float64_zero;
+ d->ZMM_D(1) = (mask & (1 << 1)) ? temp2 : float64_zero;
}
void glue(helper_mpsadbw, SUFFIX)(CPUX86State *env, Reg *d, Reg *s,
--
2.37.1
- [PATCH] i386: DPPS rounding fix,
Paolo Bonzini <=