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[3/17] Implement TARGET_C_EXCESS_PRECISION for s390

Message ID 1478878647-22547-4-git-send-email-james.greenhalgh@arm.com
State New
Headers show

Commit Message

James Greenhalgh Nov. 11, 2016, 3:37 p.m. UTC 
---
This patch has already been approved:
  https://gcc.gnu.org/ml/gcc-patches/2016-10/msg01554.html
---

This patch ports the logic from s390's TARGET_FLT_EVAL_METHOD to the new
target hook TARGET_C_EXCESS_PRECISION, following the guidance in this thread
https://gcc.gnu.org/ml/gcc-patches/2016-10/msg01172.html

In this patch we handle EXCESS_PRECISION_TYPE_IMPLICIT like
EXCESS_PRECISION_TYPE_FAST. This has the consequence of meaning float_t in
glibc is calculated in a different precision (double) to that indicated by
FLT_EVAL_METHOD.

---
gcc/

2016-11-09  James Greenhalgh  <james.greenhalgh@arm.com>

	* config/s390/s390.c (s390_excess_precision): New.
	(TARGET_C_EXCESS_PRECISION): Define.
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Patch

diff --git a/gcc/config/s390/s390.c b/gcc/config/s390/s390.c
index 3f98cd8..311a7f2 100644
--- a/gcc/config/s390/s390.c
+++ b/gcc/config/s390/s390.c
@@ -15260,6 +15260,43 @@  s390_invalid_binary_op (int op ATTRIBUTE_UNUSED, const_tree type1, const_tree ty
   return NULL;
 }
 
+/* Implement TARGET_C_EXCESS_PRECISION.
+
+   FIXME: For historical reasons, float_t and double_t are typedef'ed to
+   double on s390, causing operations on float_t to operate in a higher
+   precision than is necessary.  However, it is not the case that SFmode
+   operations have implicit excess precision, and we generate more optimal
+   code if we let the compiler know no implicit extra precision is added.
+
+   That means when we are compiling with -fexcess-precision=fast, the value
+   we set for FLT_EVAL_METHOD will be out of line with the actual precision of
+   float_t (though they would be correct for -fexcess-precision=standard).
+
+   A complete fix would modify glibc to remove the unnecessary typedef
+   of float_t to double.  */
+
+static enum flt_eval_method
+s390_excess_precision (enum excess_precision_type type)
+{
+  switch (type)
+    {
+      case EXCESS_PRECISION_TYPE_IMPLICIT:
+      case EXCESS_PRECISION_TYPE_FAST:
+	/* The fastest type to promote to will always be the native type,
+	   whether that occurs with implicit excess precision or
+	   otherwise.  */
+	return FLT_EVAL_METHOD_PROMOTE_TO_FLOAT;
+      case EXCESS_PRECISION_TYPE_STANDARD:
+	/* Otherwise, when we are in a standards compliant mode, to
+	   ensure consistency with the implementation in glibc, report that
+	   float is evaluated to the range and precision of double.  */
+	return FLT_EVAL_METHOD_PROMOTE_TO_DOUBLE;
+      default:
+	gcc_unreachable ();
+    }
+  return FLT_EVAL_METHOD_UNPREDICTABLE;
+}
+
 /* Initialize GCC target structure.  */
 
 #undef  TARGET_ASM_ALIGNED_HI_OP
@@ -15320,6 +15357,9 @@  s390_invalid_binary_op (int op ATTRIBUTE_UNUSED, const_tree type1, const_tree ty
 #undef TARGET_ASM_CAN_OUTPUT_MI_THUNK
 #define TARGET_ASM_CAN_OUTPUT_MI_THUNK hook_bool_const_tree_hwi_hwi_const_tree_true
 
+#undef TARGET_C_EXCESS_PRECISION
+#define TARGET_C_EXCESS_PRECISION s390_excess_precision
+
 #undef  TARGET_SCHED_ADJUST_PRIORITY
 #define TARGET_SCHED_ADJUST_PRIORITY s390_adjust_priority
 #undef TARGET_SCHED_ISSUE_RATE