[bootstrap-O3] use unsigned type for regno in df-scan

This patch fixes a false-positive warning in df-scan, at bootstrap-O3
failed, and enables GCC to optimize out the code that leads to the
warning.

df_ref_create_structure was inlined into the else part of
df_ref_record.  Due to the condition of the corresponding if, In the
else part, VRP deduced unsigned regno >= FIRST_PSEUDO_REGISTER.

In df_ref_create_structure, there's another regno variable,
initialized with the same expression and value as the caller's.  GCC
can tell as much, but this regno variable is signed.  It is used,
shifted right, to index a hard regset bit array within a path that
tests that this signed regno < FIRST_PSEUDO_REGISTER.

GCC warned about the possible out-of-range indexing into the hard
regset array.  It shouldn't, after all, the same regno can't possibly
be both < FIRST_PSEUDO_REGISTER and >= FIRST_PSEUDO_REGISTER, can it?

Well, the optimizers correctly decide it could, if it was a negative
int that, when converted to unsigned, became larger than
FIRST_PSEUDO_REGISTER.  But GCC doesn't know regno can't be negative,
so the test could not be optimize out.  What's more, given the
constraints, VRP correctly concluded the hard regset array would
always be indexed by a value way outside the array index range.

This patch changes the inlined regno to unsigned, like the caller's,
so that we can now tell the conditions can't both hold, so we optimize
out the path containing the would-be out-of-range array indexing.

for  gcc/ChangeLog

	* df-scan.c (df_ref_create_structure): Make regno unsigned,
	to match the caller.

From-SVN: r244090

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 1031a436aa8bee031e7b581f8b74adf2c4298de3..88fb3b4b2b53bb2aa1ab74f7bcd8c4a3a39d1831 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+2017-01-04  Alexandre Oliva <aoliva@redhat.com>
+
+       * df-scan.c (df_ref_create_structure): Make regno unsigned,
+       to match the caller.
+
 2017-01-04  Alexandre Oliva <aoliva@redhat.com>
 
        * cfgexpand.c (expand_gimple_basic_block): Disregard debug

diff --git a/gcc/df-scan.c b/gcc/df-scan.c
index ff9d5470015b124836a4a329e5ecbd614618f94c..f75098c2bec94ba472e467c7da0f04c4e1af77d1 100644 (file)
--- a/gcc/df-scan.c
+++ b/gcc/df-scan.c
@@ -2483,7 +2483,7 @@ df_ref_create_structure (enum df_ref_class cl,
                         int ref_flags)
 {
   df_ref this_ref = NULL;
-  int regno = REGNO (GET_CODE (reg) == SUBREG ? SUBREG_REG (reg) : reg);
+  unsigned int regno = REGNO (GET_CODE (reg) == SUBREG ? SUBREG_REG (reg) : reg);
   struct df_scan_problem_data *problem_data
     = (struct df_scan_problem_data *) df_scan->problem_data;