fpu: Process float_muladd_negate_result after rounding

Changing the sign before rounding affects the correctness of
the asymmetric rouding modes: float_round_up and float_round_down.

Reported-by: WANG Rui <wangrui@loongson.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

fpu/softfloat-parts.c.inc

@@ -708,10 +708,6 @@ static FloatPartsN *partsN(muladd_scalbn)(FloatPartsN *a, FloatPartsN *b,
  return_normal:
     a->exp += scale;
  finish_sign:
-    if (flags & float_muladd_negate_result) {
-        a->sign ^= 1;
-    }
-
     /*
      * All result types except for "return the default NaN
      * because this is an Invalid Operation" go through here;

fpu/softfloat.c

@@ -1731,11 +1731,8 @@ static float64 float64_round_pack_canonical(FloatParts64 *p,
     return float64_pack_raw(p);
 }
 
-static float64 float64r32_round_pack_canonical(FloatParts64 *p,
-                                               float_status *s)
+static float64 float64r32_pack_raw(FloatParts64 *p)
 {
-    parts_uncanon(p, s, &float32_params);
-
     /*
      * In parts_uncanon, we placed the fraction for float32 at the lsb.
      * We need to adjust the fraction higher so that the least N bits are
@@ -1776,6 +1773,13 @@ static float64 float64r32_round_pack_canonical(FloatParts64 *p,
     return float64_pack_raw(p);
 }
 
+static float64 float64r32_round_pack_canonical(FloatParts64 *p,
+                                               float_status *s)
+{
+    parts_uncanon(p, s, &float32_params);
+    return float64r32_pack_raw(p);
+}
+
 static void float128_unpack_canonical(FloatParts128 *p, float128 f,
                                       float_status *s)
 {
@@ -2240,7 +2244,12 @@ float16_muladd_scalbn(float16 a, float16 b, float16 c,
     float16_unpack_canonical(&pc, c, status);
     pr = parts_muladd_scalbn(&pa, &pb, &pc, scale, flags, status);
 
-    return float16_round_pack_canonical(pr, status);
+    /* Round before applying negate result. */
+    parts_uncanon(pr, status, &float16_params);
+    if ((flags & float_muladd_negate_result) && !is_nan(pr->cls)) {
+        pr->sign ^= 1;
+    }
+    return float16_pack_raw(pr);
 }
 
 float16 float16_muladd(float16 a, float16 b, float16 c,
@@ -2260,7 +2269,12 @@ float32_muladd_scalbn(float32 a, float32 b, float32 c,
     float32_unpack_canonical(&pc, c, status);
     pr = parts_muladd_scalbn(&pa, &pb, &pc, scale, flags, status);
 
-    return float32_round_pack_canonical(pr, status);
+    /* Round before applying negate result. */
+    parts_uncanon(pr, status, &float32_params);
+    if ((flags & float_muladd_negate_result) && !is_nan(pr->cls)) {
+        pr->sign ^= 1;
+    }
+    return float32_pack_raw(pr);
 }
 
 float64 QEMU_SOFTFLOAT_ATTR
@@ -2274,7 +2288,12 @@ float64_muladd_scalbn(float64 a, float64 b, float64 c,
     float64_unpack_canonical(&pc, c, status);
     pr = parts_muladd_scalbn(&pa, &pb, &pc, scale, flags, status);
 
-    return float64_round_pack_canonical(pr, status);
+    /* Round before applying negate result. */
+    parts_uncanon(pr, status, &float64_params);
+    if ((flags & float_muladd_negate_result) && !is_nan(pr->cls)) {
+        pr->sign ^= 1;
+    }
+    return float64_pack_raw(pr);
 }
 
 static bool force_soft_fma;
@@ -2428,7 +2447,12 @@ float64 float64r32_muladd(float64 a, float64 b, float64 c,
     float64_unpack_canonical(&pc, c, status);
     pr = parts_muladd_scalbn(&pa, &pb, &pc, 0, flags, status);
 
-    return float64r32_round_pack_canonical(pr, status);
+    /* Round before applying negate result. */
+    parts_uncanon(pr, status, &float32_params);
+    if ((flags & float_muladd_negate_result) && !is_nan(pr->cls)) {
+        pr->sign ^= 1;
+    }
+    return float64r32_pack_raw(pr);
 }
 
 bfloat16 QEMU_FLATTEN bfloat16_muladd(bfloat16 a, bfloat16 b, bfloat16 c,
@@ -2441,7 +2465,12 @@ bfloat16 QEMU_FLATTEN bfloat16_muladd(bfloat16 a, bfloat16 b, bfloat16 c,
     bfloat16_unpack_canonical(&pc, c, status);
     pr = parts_muladd_scalbn(&pa, &pb, &pc, 0, flags, status);
 
-    return bfloat16_round_pack_canonical(pr, status);
+    /* Round before applying negate result. */
+    parts_uncanon(pr, status, &bfloat16_params);
+    if ((flags & float_muladd_negate_result) && !is_nan(pr->cls)) {
+        pr->sign ^= 1;
+    }
+    return bfloat16_pack_raw(pr);
 }
 
 float128 QEMU_FLATTEN float128_muladd(float128 a, float128 b, float128 c,
@@ -2454,7 +2483,12 @@ float128 QEMU_FLATTEN float128_muladd(float128 a, float128 b, float128 c,
     float128_unpack_canonical(&pc, c, status);
     pr = parts_muladd_scalbn(&pa, &pb, &pc, 0, flags, status);
 
-    return float128_round_pack_canonical(pr, status);
+    /* Round before applying negate result. */
+    parts_uncanon(pr, status, &float128_params);
+    if ((flags & float_muladd_negate_result) && !is_nan(pr->cls)) {
+        pr->sign ^= 1;
+    }
+    return float128_pack_raw(pr);
 }
 
 /*

tests/tcg/multiarch/Makefile.target

@@ -29,6 +29,7 @@ run-float_%: float_%
 	$(call run-test,$<, $(QEMU) $(QEMU_OPTS) $<)
 	$(call conditional-diff-out,$<,$(SRC_PATH)/tests/tcg/$(TARGET_NAME)/$<.ref)
 
+fnmsub: LDFLAGS+=-lm
 
 testthread: LDFLAGS+=-lpthread
 

tests/tcg/multiarch/fnmsub.c

@@ -0,0 +1,37 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include <stdio.h>
+#include <math.h>
+#include <fenv.h>
+
+union U {
+  double d;
+  unsigned long long l;
+};
+
+union U x = { .l = 0x4ff0000000000000ULL };
+union U y = { .l = 0x2ff0000000000000ULL };
+union U r;
+
+int main()
+{
+#ifdef FE_DOWNWARD
+    fesetround(FE_DOWNWARD);
+
+#if defined(__loongarch__)
+    asm("fnmsub.d %0, %1, %1, %2" : "=f"(r.d) : "f"(x.d), "f"(y.d));
+#elif defined(__powerpc64__)
+    asm("fnmsub %0,%1,%1,%2" : "=f"(r.d) : "f"(x.d), "f"(y.d));
+#elif defined(__s390x__) && 0 /* need -march=z14 */
+    asm("vfnms %0,%1,%1,%2,0,3" : "=f"(r.d) : "f"(x.d), "f"(y.d));
+#else
+    r.d = -fma(x.d, x.d, -y.d);
+#endif
+
+    if (r.l != 0xdfefffffffffffffULL) {
+        printf("r = %.18a (%016llx)\n", r.d, r.l);
+        return 1;
+    }
+#endif
+    return 0;
+}