i386: Don't emit fldpi etc. if -frounding-math [PR102498]

i387 has instructions to store some transcedental numbers into the top of
stack.  The problem is that what exact bit in the last place one gets for
those depends on the current rounding mode, the CPU knows the number with
slightly higher precision.  The compiler assumes rounding to nearest when
comparing them against constants in the IL, but at runtime the rounding
can be different and so some of these depending on rounding mode and the
constant could be 1 ulp higher or smaller than expected.
We only support changing the rounding mode at runtime if the non-default
-frounding-mode option is used, so the following patch just disables
using those constants if that flag is on.

2021-09-28  Jakub Jelinek  <jakub@redhat.com>

	PR target/102498
	* config/i386/i386.c (standard_80387_constant_p): Don't recognize
	special 80387 instruction XFmode constants if flag_rounding_math.

	* gcc.target/i386/pr102498.c: New test.

(cherry picked from commit 3b7041e8345c2f1030e58620f28e22d64b2c196b)

diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index 527d493ecae96a55ee6c1a3f76a52508d9daae8a..f991e07f232d91e95b6788d369c39a73de626ef0 100644 (file)
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -4963,7 +4963,8 @@ standard_80387_constant_p (rtx x)
   /* For XFmode constants, try to find a special 80387 instruction when
      optimizing for size or on those CPUs that benefit from them.  */
   if (mode == XFmode
-      && (optimize_function_for_size_p (cfun) || TARGET_EXT_80387_CONSTANTS))
+      && (optimize_function_for_size_p (cfun) || TARGET_EXT_80387_CONSTANTS)
+      && !flag_rounding_math)
     {
       int i;
 

diff --git a/gcc/testsuite/gcc.target/i386/pr102498.c b/gcc/testsuite/gcc.target/i386/pr102498.c
new file mode 100644 (file)
index 0000000..44161c5
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr102498.c
@@ -0,0 +1,59 @@
+/* PR target/102498 */
+/* { dg-do run { target fenv } } */
+/* { dg-options "-frounding-math" } */
+
+#include <fenv.h>
+#include <stdlib.h>
+
+__attribute__((noipa)) long double
+fldlg2 (void)
+{
+  return 0.3010299956639811952256464283594894482L;
+}
+
+__attribute__((noipa)) long double
+fldln2 (void)
+{
+  return 0.6931471805599453094286904741849753009L;
+}
+
+__attribute__((noipa)) long double
+fldl2e (void)
+{
+  return 1.4426950408889634073876517827983434472L;
+}
+
+__attribute__((noipa)) long double
+fldl2t (void)
+{
+  return 3.3219280948873623478083405569094566090L;
+}
+
+__attribute__((noipa)) long double
+fldpi (void)
+{
+  return 3.1415926535897932385128089594061862044L;
+}
+
+int
+main ()
+{
+  long double a = fldlg2 ();
+  long double b = fldln2 ();
+  long double c = fldl2e ();
+  long double d = fldl2t ();
+  long double e = fldpi ();
+  static int f[] = { FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD, FE_DOWNWARD };
+  int i;
+  for (i = 0; i < 4; i++)
+    {
+      fesetround (f[i]);
+      if (a != fldlg2 ()
+         || b != fldln2 ()
+         || c != fldl2e ()
+         || d != fldl2t ()
+         || e != fldpi ())
+       abort ();
+    }
+  return 0;
+}