[gcc r9-9126] i386: Fix __builtin_floor with FE_DOWNWARD rounding direction [PR96793]

[gcc r9-9126] i386: Fix __builtin_floor with FE_DOWNWARD rounding direction [PR96793]

[Uros Bizjak]

https://gcc.gnu.org/g:c40b640ebcef1aae78eaca56e04d204dda9e4cad

commit r9-9126-gc40b640ebcef1aae78eaca56e04d204dda9e4cad
Author: Uros Bizjak <ubizjak@gmail.com>
Date:   Wed Dec 23 09:09:29 2020 +0100

    i386: Fix __builtin_floor with FE_DOWNWARD rounding direction [PR96793]
    
    x86_expand_floorceil expander uses x86_sse_copysign_to_positive, which
    is unable to change the sign from - to +.  When FE_DOWNWARD rounding
    direction is in effect, the expanded sequence that involves subtraction
    can trigger x - x = -0.0 special rule.  x86_sse_copysign_to_positive
    fails to change the sign of the intermediate value, assumed to always
    be positive, back to positive.
    
    The patch adds one extra fabs that strips the sign from the intermediate
    value when flag_rounding_math is in effect.
    
    2020-12-22  Uroš Bizjak  <ubizjak@gmail.com>
    
    gcc/
            PR target/96793
            * config/i386/i386.c (ix86_expand_floorceil):
            Remove the sign of the intermediate value for flag_rounding_math.
            (ix86_expand_floorceildf_32): Ditto.
    
    gcc/testsuite/
            PR target/96793
            * gcc.target/i386/pr96793.c: New test.

Diff:
---
 gcc/config/i386/i386.c                  | 25 ++++++++++++++++++++-----
 gcc/testsuite/gcc.target/i386/pr96793.c | 28 ++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+), 5 deletions(-)

diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index e8982c507e4..e0f924e50b2 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -45511,12 +45511,14 @@ ix86_expand_floorceildf_32 (rtx operand0, rtx operand1, bool do_floor)
           return x;
         xa = xa + TWO52 - TWO52;
         x2 = copysign (xa, x);
+
      Compensate.  Floor:
         if (x2 > x)
           x2 -= 1;
      Compensate.  Ceil:
         if (x2 < x)
           x2 += 1;
+
 	if (HONOR_SIGNED_ZEROS (mode))
 	  x2 = copysign (x2, x);
 	return x2;
@@ -45553,8 +45555,14 @@ ix86_expand_floorceildf_32 (rtx operand0, rtx operand1, bool do_floor)
   emit_insn (gen_rtx_SET (tmp, gen_rtx_AND (mode, one, tmp)));
   tmp = expand_simple_binop (mode, do_floor ? MINUS : PLUS,
 			     xa, tmp, NULL_RTX, 0, OPTAB_DIRECT);
-  if (!do_floor && HONOR_SIGNED_ZEROS (mode))
-    ix86_sse_copysign_to_positive (tmp, tmp, res, mask);
+  if (HONOR_SIGNED_ZEROS (mode))
+    {
+      /* Remove the sign with FE_DOWNWARD, where x - x = -0.0.  */
+      if (do_floor && flag_rounding_math)
+	tmp = ix86_expand_sse_fabs (tmp, NULL);
+
+      ix86_sse_copysign_to_positive (tmp, tmp, res, mask);
+    }
   emit_move_insn (res, tmp);
 
   emit_label (label);
@@ -45573,12 +45581,14 @@ ix86_expand_floorceil (rtx operand0, rtx operand1, bool do_floor)
         if (!isless (xa, TWO52))
           return x;
 	x2 = (double)(long)x;
+
      Compensate.  Floor:
 	if (x2 > x)
 	  x2 -= 1;
      Compensate.  Ceil:
 	if (x2 < x)
 	  x2 += 1;
+
 	if (HONOR_SIGNED_ZEROS (mode))
 	  return copysign (x2, x);
 	return x2;
@@ -45613,10 +45623,15 @@ ix86_expand_floorceil (rtx operand0, rtx operand1, bool do_floor)
   emit_insn (gen_rtx_SET (tmp, gen_rtx_AND (mode, one, tmp)));
   tmp = expand_simple_binop (mode, do_floor ? MINUS : PLUS,
 			     xa, tmp, NULL_RTX, 0, OPTAB_DIRECT);
-  emit_move_insn (res, tmp);
-
   if (HONOR_SIGNED_ZEROS (mode))
-    ix86_sse_copysign_to_positive (res, res, force_reg (mode, operand1), mask);
+    {
+      /* Remove the sign with FE_DOWNWARD, where x - x = -0.0.  */
+      if (do_floor && flag_rounding_math)
+	tmp = ix86_expand_sse_fabs (tmp, NULL);
+
+      ix86_sse_copysign_to_positive (tmp, tmp, res, mask);
+    }
+  emit_move_insn (res, tmp);
 
   emit_label (label);
   LABEL_NUSES (label) = 1;
diff --git a/gcc/testsuite/gcc.target/i386/pr96793.c b/gcc/testsuite/gcc.target/i386/pr96793.c
new file mode 100644
index 00000000000..4a96478e31e
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr96793.c
@@ -0,0 +1,28 @@
+/* PR target/96793 */
+/* { dg-do run { target sse2_runtime } } */
+/* { dg-require-effective-target fenv } */
+/* { dg-options "-O2 -frounding-math -msse2 -mno-sse4 -mfpmath=sse" } */
+
+#include <fenv.h>
+
+double
+__attribute__((noinline))
+test (double value)
+{
+  return __builtin_floor (value);
+}
+
+int
+main ()
+{
+  double result;
+
+  fesetround (FE_DOWNWARD);
+
+  result = test (0.25);
+
+  if (__builtin_signbit (result) != 0)
+    __builtin_abort ();
+
+  return 0;
+}