diff mbox series

[PULL,5/7] target/i386: Do not raise Invalid for 0 * Inf + QNaN

Message ID 20250207102802.2445596-6-pbonzini@redhat.com
State New
Headers show
Series None | expand

Commit Message

Paolo Bonzini Feb. 7, 2025, 10:27 a.m. UTC 
From: Peter Maydell <peter.maydell@linaro.org>

In commit 8adcff4ae7 ("fpu: handle raising Invalid for infzero in
pick_nan_muladd") we changed the handling of 0 * Inf + QNaN to always
raise the Invalid exception regardless of target architecture.  (This
was a change affecting hppa, i386, sh4 and tricore.) However, this
was incorrect for i386, which documents in the SDM section 14.5.2
that for the 0 * Inf + NaN case that it will only raise the Invalid
exception when the input is an SNaN.  (This is permitted by the IEEE
754-2008 specification, which documents that whether we raise Invalid
for 0 * Inf + QNaN is implementation defined.)

Adjust the softfloat pick_nan_muladd code to allow the target to
suppress the raising of Invalid for the inf * zero + NaN case (as an
extra flag orthogonal to its choice for when to use the default NaN),
and enable that for x86.

We do not revert here the behaviour change for hppa, sh4 or tricore:
 * The sh4 manual is clear that it should signal Invalid
 * The tricore manual is a bit vague but doesn't say it shouldn't
 * The hppa manual doesn't talk about fused multiply-add corner
   cases at all

Cc: qemu-stable@nongnu.org
Fixes: 8adcff4ae7 (""fpu: handle raising Invalid for infzero in pick_nan_muladd")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Link: https://lore.kernel.org/r/20250116112536.4117889-2-peter.maydell@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 include/fpu/softfloat-types.h | 16 +++++++++++++---
 target/i386/tcg/fpu_helper.c  |  5 ++++-
 fpu/softfloat-parts.c.inc     |  5 +++--
 3 files changed, 20 insertions(+), 6 deletions(-)

Comments

Michael Tokarev Feb. 7, 2025, 11:53 a.m. UTC | #1 
07.02.2025 13:27, Paolo Bonzini wrote:
> From: Peter Maydell <peter.maydell@linaro.org>
> 
> In commit 8adcff4ae7 ("fpu: handle raising Invalid for infzero in
> pick_nan_muladd") we changed the handling of 0 * Inf + QNaN to always
> raise the Invalid exception regardless of target architecture.  (This
> was a change affecting hppa, i386, sh4 and tricore.) However, this
> was incorrect for i386, which documents in the SDM section 14.5.2
> that for the 0 * Inf + NaN case that it will only raise the Invalid
> exception when the input is an SNaN.  (This is permitted by the IEEE
> 754-2008 specification, which documents that whether we raise Invalid
> for 0 * Inf + QNaN is implementation defined.)
> 
> Adjust the softfloat pick_nan_muladd code to allow the target to
> suppress the raising of Invalid for the inf * zero + NaN case (as an
> extra flag orthogonal to its choice for when to use the default NaN),
> and enable that for x86.
> 
> We do not revert here the behaviour change for hppa, sh4 or tricore:
>   * The sh4 manual is clear that it should signal Invalid
>   * The tricore manual is a bit vague but doesn't say it shouldn't
>   * The hppa manual doesn't talk about fused multiply-add corner
>     cases at all
> 
> Cc: qemu-stable@nongnu.org
> Fixes: 8adcff4ae7 (""fpu: handle raising Invalid for infzero in pick_nan_muladd")

A nitpick: double double-quote.

8adcff4ae7 is v9.2.0-7-g8adcff4ae7 - which is 7 commits *after* the latest
released version, -- hopefully this fix should not go to any stable series,
unless 8adcff4ae7 itself has to be picked up for 9.2 too.

Thanks,

/mjt
Peter Maydell Feb. 7, 2025, 1:43 p.m. UTC | #2 
On Fri, 7 Feb 2025 at 11:53, Michael Tokarev <mjt@tls.msk.ru> wrote:
>
> 07.02.2025 13:27, Paolo Bonzini wrote:
> > From: Peter Maydell <peter.maydell@linaro.org>
> >
> > In commit 8adcff4ae7 ("fpu: handle raising Invalid for infzero in
> > pick_nan_muladd") we changed the handling of 0 * Inf + QNaN to always
> > raise the Invalid exception regardless of target architecture.  (This
> > was a change affecting hppa, i386, sh4 and tricore.) However, this
> > was incorrect for i386, which documents in the SDM section 14.5.2
> > that for the 0 * Inf + NaN case that it will only raise the Invalid
> > exception when the input is an SNaN.  (This is permitted by the IEEE
> > 754-2008 specification, which documents that whether we raise Invalid
> > for 0 * Inf + QNaN is implementation defined.)
> >
> > Adjust the softfloat pick_nan_muladd code to allow the target to
> > suppress the raising of Invalid for the inf * zero + NaN case (as an
> > extra flag orthogonal to its choice for when to use the default NaN),
> > and enable that for x86.
> >
> > We do not revert here the behaviour change for hppa, sh4 or tricore:
> >   * The sh4 manual is clear that it should signal Invalid
> >   * The tricore manual is a bit vague but doesn't say it shouldn't
> >   * The hppa manual doesn't talk about fused multiply-add corner
> >     cases at all
> >
> > Cc: qemu-stable@nongnu.org
> > Fixes: 8adcff4ae7 (""fpu: handle raising Invalid for infzero in pick_nan_muladd")
>
> A nitpick: double double-quote.
>
> 8adcff4ae7 is v9.2.0-7-g8adcff4ae7 - which is 7 commits *after* the latest
> released version, -- hopefully this fix should not go to any stable series,
> unless 8adcff4ae7 itself has to be picked up for 9.2 too.

Ah, yes, I think I assumed based on date that 8adcff4ae7 had
made it into a release already. That commit was a refactoring
so it doesn't need to be backported anywhere.

-- PMM
diff mbox series

Patch

diff --git a/include/fpu/softfloat-types.h b/include/fpu/softfloat-types.h
index 616c290145f..2e43d1dd9e6 100644
--- a/include/fpu/softfloat-types.h
+++ b/include/fpu/softfloat-types.h
@@ -280,11 +280,21 @@  typedef enum __attribute__((__packed__)) {
     /* No propagation rule specified */
     float_infzeronan_none = 0,
     /* Result is never the default NaN (so always the input NaN) */
-    float_infzeronan_dnan_never,
+    float_infzeronan_dnan_never = 1,
     /* Result is always the default NaN */
-    float_infzeronan_dnan_always,
+    float_infzeronan_dnan_always = 2,
     /* Result is the default NaN if the input NaN is quiet */
-    float_infzeronan_dnan_if_qnan,
+    float_infzeronan_dnan_if_qnan = 3,
+    /*
+     * Don't raise Invalid for 0 * Inf + NaN. Default is to raise.
+     * IEEE 754-2008 section 7.2 makes it implementation defined whether
+     * 0 * Inf + QNaN raises Invalid or not. Note that 0 * Inf + SNaN will
+     * raise the Invalid flag for the SNaN anyway.
+     *
+     * This is a flag which can be ORed in with any of the above
+     * DNaN behaviour options.
+     */
+    float_infzeronan_suppress_invalid = (1 << 7),
 } FloatInfZeroNaNRule;
 
 /*
diff --git a/target/i386/tcg/fpu_helper.c b/target/i386/tcg/fpu_helper.c
index 3d764bc138d..de6d0b252ec 100644
--- a/target/i386/tcg/fpu_helper.c
+++ b/target/i386/tcg/fpu_helper.c
@@ -178,8 +178,11 @@  void cpu_init_fp_statuses(CPUX86State *env)
      * "Fused-Multiply-ADD (FMA) Numeric Behavior" the NaN handling is
      * specified -- for 0 * inf + NaN the input NaN is selected, and if
      * there are multiple input NaNs they are selected in the order a, b, c.
+     * We also do not raise Invalid for the 0 * inf + (Q)NaN case.
      */
-    set_float_infzeronan_rule(float_infzeronan_dnan_never, &env->sse_status);
+    set_float_infzeronan_rule(float_infzeronan_dnan_never |
+                              float_infzeronan_suppress_invalid,
+                              &env->sse_status);
     set_float_3nan_prop_rule(float_3nan_prop_abc, &env->sse_status);
     /* Default NaN: sign bit set, most significant frac bit set */
     set_float_default_nan_pattern(0b11000000, &env->fp_status);
diff --git a/fpu/softfloat-parts.c.inc b/fpu/softfloat-parts.c.inc
index fee05d0a863..73621f4a970 100644
--- a/fpu/softfloat-parts.c.inc
+++ b/fpu/softfloat-parts.c.inc
@@ -126,7 +126,8 @@  static FloatPartsN *partsN(pick_nan_muladd)(FloatPartsN *a, FloatPartsN *b,
         float_raise(float_flag_invalid | float_flag_invalid_snan, s);
     }
 
-    if (infzero) {
+    if (infzero &&
+        !(s->float_infzeronan_rule & float_infzeronan_suppress_invalid)) {
         /* This is (0 * inf) + NaN or (inf * 0) + NaN */
         float_raise(float_flag_invalid | float_flag_invalid_imz, s);
     }
@@ -144,7 +145,7 @@  static FloatPartsN *partsN(pick_nan_muladd)(FloatPartsN *a, FloatPartsN *b,
          * Inf * 0 + NaN -- some implementations return the
          * default NaN here, and some return the input NaN.
          */
-        switch (s->float_infzeronan_rule) {
+        switch (s->float_infzeronan_rule & ~float_infzeronan_suppress_invalid) {
         case float_infzeronan_dnan_never:
             break;
         case float_infzeronan_dnan_always: