[PULL 055/115] target/i386: fix fscale handling of rounding precision

[Paolo Bonzini]

From: Joseph Myers <joseph@codesourcery.com>

The fscale implementation uses floatx80_scalbn for the final scaling
operation.  floatx80_scalbn ends up rounding the result using the
dynamic rounding precision configured for the FPU.  But only a limited
set of x87 floating-point instructions are supposed to respect the
dynamic rounding precision, and fscale is not in that set.  Fix the
implementation to save and restore the rounding precision around the
call to floatx80_scalbn.

Signed-off-by: Joseph Myers <joseph@codesourcery.com>
Message-Id: <alpine.DEB.2.21.2005070045430.18350@digraph.polyomino.org.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/fpu_helper.c          |  3 +++
 tests/tcg/i386/test-i386-fscale.c | 13 +++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/target/i386/fpu_helper.c b/target/i386/fpu_helper.c
index 72d1e77eb0..4d14c1ca24 100644
--- a/target/i386/fpu_helper.c
+++ b/target/i386/fpu_helper.c
@@ -1001,7 +1001,10 @@ void helper_fscale(CPUX86State *env)
         }
     } else {
         int n = floatx80_to_int32_round_to_zero(ST1, &env->fp_status);
+        signed char save = env->fp_status.floatx80_rounding_precision;
+        env->fp_status.floatx80_rounding_precision = 80;
         ST0 = floatx80_scalbn(ST0, n, &env->fp_status);
+        env->fp_status.floatx80_rounding_precision = save;
     }
 }
 
diff --git a/tests/tcg/i386/test-i386-fscale.c 
b/tests/tcg/i386/test-i386-fscale.c
index b953e7c563..d23b3cfeec 100644
--- a/tests/tcg/i386/test-i386-fscale.c
+++ b/tests/tcg/i386/test-i386-fscale.c
@@ -8,6 +8,8 @@ union u {
     long double ld;
 };
 
+volatile long double ld_third = 1.0L / 3.0L;
+volatile long double ld_four_thirds = 4.0L / 3.0L;
 volatile union u ld_invalid_1 = { .s = { 1, 1234 } };
 volatile union u ld_invalid_2 = { .s = { 0, 1234 } };
 volatile union u ld_invalid_3 = { .s = { 0, 0x7fff } };
@@ -91,5 +93,16 @@ int main(void)
         printf("FAIL: fscale finite down inf\n");
         ret = 1;
     }
+    /* Set round-to-nearest with single-precision rounding.  */
+    cw = cw & ~0xf00;
+    __asm__ volatile ("fldcw %0" : : "m" (cw));
+    __asm__ volatile ("fscale" : "=t" (ld_res) :
+                      "0" (ld_third), "u" (2.0L));
+    cw = cw | 0x300;
+    __asm__ volatile ("fldcw %0" : : "m" (cw));
+    if (ld_res != ld_four_thirds) {
+        printf("FAIL: fscale single-precision\n");
+        ret = 1;
+    }
     return ret;
 }
-- 
2.26.2