[PATCH][ARM] PR target/67929 Tighten vfp3_const_double_for_bits checks

[Kyrill Tkachov <kyrylo dot tkachov at arm dot com>]

Hi all,

This patch fixes the referenced PR by rewriting the vfp3_const_double_for_bits function in arm.c
The function is supposed to accept positive CONST_DOUBLE rtxes whose value is an exact power of 2
and whose log2 is between 1 and 32. That is values like 2.0, 4.0, 8.9, 16.0 etc...

The current implementation seems to have been written under the assumption that exact_real_truncate returns
false if the input value is not an exact integer, whereas in fact exact_real_truncate returns false if the
truncation operation was not exact, which are different things. This would lead the function to accept any
CONST_DOUBLE that can truncate to a power of 2, such as 4.9, 16.2 etc.

In any case, I've rewritten this function and used the real_isinteger predicate to check if the real value
is an exact integer.

The testcase demonstrates the kind of wrong code that this patch addresses.

This bug appears on GCC 5 and 4.9 as well, but due to the recent introduction of CONST_DOUBLE_REAL_VALUE
this patch doesn't apply on those branches. I will soon post the backportable variant.

Bootstrapped and tested on arm-none-linux-gnueabihf.

Ok for trunk?

Thanks,
Kyrill

2015-10-12  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    PR target/67929
    * config/arm/arm.c (vfp3_const_double_for_bits): Rewrite.
    * config/arm/constraints.md (Dp): Update callsite.
    * config/arm/predicates.md (const_double_vcvt_power_of_two): Likewise.

2015-10-12  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    PR target/67929
    * gcc.target/arm/pr67929_1.c: New test.

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 0bf1164..29dd489 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -27734,25 +27734,37 @@ vfp3_const_double_for_fract_bits (rtx operand)
   return 0;
 }
 
+/* If X is a CONST_DOUBLE with a value that is a power of 2 whose
+   log2 is in [1, 32], return that log2.  Otherwise return -1.
+   This is used in the patterns for vcvt.s32.f32 floating-point to
+   fixed-point conversions.  */
+
 int
-vfp3_const_double_for_bits (rtx operand)
+vfp3_const_double_for_bits (rtx x)
 {
-  const REAL_VALUE_TYPE *r0;
+  const REAL_VALUE_TYPE *r;
 
-  if (!CONST_DOUBLE_P (operand))
-    return 0;
+  if (!CONST_DOUBLE_P (x))
+    return -1;
 
-  r0 = CONST_DOUBLE_REAL_VALUE (operand);
-  if (exact_real_truncate (DFmode, r0))
-    {
-      HOST_WIDE_INT value = real_to_integer (r0);
-      value = value & 0xffffffff;
-      if ((value != 0) && ( (value & (value - 1)) == 0))
-	return int_log2 (value);
-    }
+  r = CONST_DOUBLE_REAL_VALUE (x);
 
-  return 0;
+  if (REAL_VALUE_NEGATIVE (*r)
+      || REAL_VALUE_ISNAN (*r)
+      || REAL_VALUE_ISINF (*r)
+      || !real_isinteger (r, SFmode))
+    return -1;
+
+  HOST_WIDE_INT hwint = exact_log2 (real_to_integer (r));
+
+/* The exact_log2 above will have returned -1 if this is
+   not an exact log2.  */
+  if (!IN_RANGE (hwint, 1, 32))
+    return -1;
+
+  return hwint;
 }
+
 
 /* Emit a memory barrier around an atomic sequence according to MODEL.  */
 
diff --git a/gcc/config/arm/constraints.md b/gcc/config/arm/constraints.md
index e24858f..901cfe5 100644
--- a/gcc/config/arm/constraints.md
+++ b/gcc/config/arm/constraints.md
@@ -339,7 +339,8 @@
  "@internal
   In ARM/ Thumb2 a const_double which can be used with a vcvt.s32.f32 with bits operation"
   (and (match_code "const_double")
-       (match_test "TARGET_32BIT && TARGET_VFP && vfp3_const_double_for_bits (op)")))
+       (match_test "TARGET_32BIT && TARGET_VFP
+		    && vfp3_const_double_for_bits (op) > 0")))
 
 (define_register_constraint "Ts" "(arm_restrict_it) ? LO_REGS : GENERAL_REGS"
  "For arm_restrict_it the core registers @code{r0}-@code{r7}.  GENERAL_REGS otherwise.")
diff --git a/gcc/config/arm/predicates.md b/gcc/config/arm/predicates.md
index 08cc899..48e4ba8 100644
--- a/gcc/config/arm/predicates.md
+++ b/gcc/config/arm/predicates.md
@@ -668,7 +668,7 @@
 (define_predicate "const_double_vcvt_power_of_two"
   (and (match_code "const_double")
        (match_test "TARGET_32BIT && TARGET_VFP
-                   && vfp3_const_double_for_bits (op)")))
+		    && vfp3_const_double_for_bits (op) > 0")))
 
 (define_predicate "neon_struct_operand"
   (and (match_code "mem")
diff --git a/gcc/testsuite/gcc.target/arm/pr67929_1.c b/gcc/testsuite/gcc.target/arm/pr67929_1.c
new file mode 100644
index 0000000..14943b6
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/pr67929_1.c
@@ -0,0 +1,21 @@
+/* { dg-do run } */
+/* { dg-require-effective-target arm_vfp3_ok } */
+/* { dg-options "-O2 -fno-inline" } */
+/* { dg-add-options arm_vfp3 } */
+/* { dg-skip-if "need fp instructions" { *-*-* } { "-mfloat-abi=soft" } { "" } } */
+
+int
+foo (float a)
+{
+  return a * 4.9f;
+}
+
+
+int
+main (void)
+{
+  if (foo (10.0f) != 49)
+    __builtin_abort ();
+
+  return 0;
+}
\ No newline at end of file