middle-end/57245 - honor -frounding-math in real truncation

The following honors -frounding-math when converting a FP constant
to another FP type.

2021-10-27  Richard Biener  <rguenther@suse.de>

	PR middle-end/57245
	* fold-const.c (fold_convert_const_real_from_real): Honor
	-frounding-math if the conversion is not exact.
	* simplify-rtx.c (simplify_const_unary_operation): Do not
	simplify FLOAT_TRUNCATE with sign dependent rounding.

	* gcc.dg/torture/fp-double-convert-float-1.c: New testcase.

3 files changed, 52 insertions, 0 deletions

diff --git a/gcc/fold-const.c b/gcc/fold-const.c
index ff23f12..18950ae 100644
--- a/gcc/fold-const.c
+++ b/gcc/fold-const.c
@@ -2139,6 +2139,12 @@ fold_convert_const_real_from_real (tree type, const_tree arg1)
       && REAL_VALUE_ISSIGNALING_NAN (TREE_REAL_CST (arg1)))
     return NULL_TREE; 
 
+  /* With flag_rounding_math we should respect the current rounding mode
+     unless the conversion is exact.  */
+  if (HONOR_SIGN_DEPENDENT_ROUNDING (arg1)
+      && !exact_real_truncate (TYPE_MODE (type), &TREE_REAL_CST (arg1)))
+    return NULL_TREE;
+
   real_convert (&value, TYPE_MODE (type), &TREE_REAL_CST (arg1));
   t = build_real (type, value);
 
diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index bbbd6b7..f38b6d7 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -2068,6 +2068,11 @@ simplify_const_unary_operation (enum rtx_code code, machine_mode mode,
 	     and the operand is a signaling NaN.  */
 	  if (HONOR_SNANS (mode) && REAL_VALUE_ISSIGNALING_NAN (d))
 	    return NULL_RTX;
+	  /* Or if flag_rounding_math is on and the truncation is not
+	     exact.  */
+	  if (HONOR_SIGN_DEPENDENT_ROUNDING (mode)
+	      && !exact_real_truncate (mode, &d))
+	    return NULL_RTX;
 	  d = real_value_truncate (mode, d);
 	  break;
 	case FLOAT_EXTEND:
diff --git a/gcc/testsuite/gcc.dg/torture/fp-double-convert-float-1.c b/gcc/testsuite/gcc.dg/torture/fp-double-convert-float-1.c
new file mode 100644
index 0000000..ec23274
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/fp-double-convert-float-1.c
@@ -0,0 +1,41 @@
+/* PR57245 */
+/* { dg-do run } */
+/* { dg-require-effective-target fenv } */
+/* { dg-additional-options "-frounding-math" } */
+
+#include <fenv.h>
+#include <stdlib.h>
+
+int
+main ()
+{
+#if __DBL_MANT_DIG__ == 53 && __FLT_MANT_DIG__ == 24
+#ifdef FE_UPWARD
+  fesetround (FE_UPWARD);
+  float f = 1.3;
+  if (f != 0x1.4ccccep+0f)
+    __builtin_abort ();
+#endif
+#ifdef FE_TONEAREST
+  fesetround (FE_TONEAREST);
+  /* Use different actual values so the bogus CSE we perform does not
+     break things.  */
+  f = 1.33;
+  if (f != 0x1.547ae2p+0f)
+    abort ();
+#endif
+#ifdef FE_DOWNWARD
+  fesetround (FE_DOWNWARD);
+  f = 1.333;
+  if (f != 0x1.553f7cp+0f)
+    abort ();
+#endif
+#ifdef FE_TOWARDZERO
+  fesetround (FE_TOWARDZERO);
+  f = 1.3333;
+  if (f != 0x1.555326p+0f)
+    abort ();
+#endif
+#endif
+  return 0;
+}