This is the mail archive of the gcc-patches@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

[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.

Regstrapped on x86_64-linux-gnu and i686-linux-gnu.  OK to install?

for  gcc/ChangeLog

	* df-scan.c (df_ref_create_structure): Make regno unsigned,
	to match the caller.
---
 gcc/df-scan.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gcc/df-scan.c b/gcc/df-scan.c
index e6b55b5..ee8282f 100644
--- 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;
 

-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist|Red Hat Brasil GNU Toolchain Engineer


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]