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]

[PATCH][ARM] Fix broken shift patterns


This patch is a follow-up both to my patches here:

http://gcc.gnu.org/ml/gcc-patches/2011-09/msg00049.html

and Paul Brook's patch here:

http://gcc.gnu.org/ml/gcc-patches/2011-09/msg01076.html

The patch fixes both the original problem, in which negative shift constants caused an ICE (pr50193), and the problem introduced by Paul's patch, in which "a*64+b" is not properly optimized.

However, it does not attempt to fix Richard Sandiford's observation that there may be a latent problem with the 'M' constraint which could lead reload to cause a recog ICE.

I believe this patch to be nothing but an improvement over the current state, and that a fix to the constraint problem should be a separate patch.

In that basis, am I OK to commit?



Now, let me explain the other problem:

As it stands, all shift-related patterns, for ARM or Thumb2 mode, permit a shift to be expressed as either a shift type and amount (register or constant), or as a multiply and power-of-two constant.

This is necessary because the canonical form of (plus (ashift x y) z) appears to be (plus (mult x 2^y) z), presumably for the benefit of multiply-and-accumulate optimizations. (Minus is similarly affected, but other shiftable operations are unaffected, and this only applies to left shifts, of course.)

The (possible) problem is that the meanings of the constants for mult and ashift are very different, but the arm.md file has these unified into a single pattern using a single 'M' constraint that must allow both types of constant unconditionally. This is safe for the vast majority of passes because they check recog before they make a change, and anyway don't make changes without understanding the logic. But, reload has a feature where it can pull a constant from a register, and convert it to an immediate, if the constraints allow, but crucially, it doesn't check the predicates; no doubt it shouldn't need to, but the ARM port appears to be breaking to rules.

Problem scenario 1:

Consider pattern (plus (mult r1 r2) r3).

  It so happens that reload knows that r2 contains a constant, say 20,
  so reload checks to see if that could be converted to an immediate.
  Now, 20 is not a power of two, so recog would reject it, but it is in
  the range 0..31 so it does match the 'M' constraint. Oops!

Problem scenario 2:

Consider pattern (ashiftrt r1 r2).

  Again, it so happens that reload knows that r2 contains a constant, in
  this case let's say 64, so again reload checks to see if that could
  be converted to an immediate. This time, 64 is not in the range
  0..31, so recog would reject it, but it is a power of two, so it does
  match the 'M' constraint. Again, oops!

I see two ways to fix this properly:

 1. Duplicate all the patterns in the machine description, once for the
    mult case, and once for the other cases. This could probably be
    done with a code iterator, if preferred.

 2. Redefine the canonical form of (plus (mult .. 2^n) ..) such that it
    always uses the (presumably cheaper) shift-and-add option. However,
    this would require all other targets where madd really is the best
    option to fix it up. (I'd imagine that two instructions for shift
    and add would be cheaper speed wise, if properly scheduled, on most
    targets? That doesn't help the size optimization though.)

However, it's not obvious to me that this needs fixing:
 * The failure mode would be an ICE, and we've not seen any.
 * There's a comment in arm.c:shift_op that suggests that this can't
   happen, somehow, at least in the mult case.
   - I'm not sure exactly how reload works, but it seems reasonable
     that it will never try to convert a register to an immediate
     because the pattern does not allow registers in the first place.
   - This logic doesn't hold in the opposite case though.

Have I explained all that clearly?

My conclusion after studying all this is that we don't need to do anything until somebody reports an ICE, at which point it becomes worth the effort of fixing it. Other opinions welcome! :)

Andrew
2011-10-06  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* config/arm/predicates.md (shift_amount_operand): Remove constant
	range check.
	(shift_operator): Check range of constants for all shift operators.

	gcc/testsuite/
	* gcc.dg/pr50193-1.c: New file.
	* gcc.target/arm/shiftable.c: New file.

---
 src/gcc-mainline/gcc/config/arm/predicates.md      |   15 ++++++-
 src/gcc-mainline/gcc/testsuite/gcc.dg/pr50193-1.c  |   10 +++++
 .../gcc/testsuite/gcc.target/arm/shiftable.c       |   43 ++++++++++++++++++++
 3 files changed, 65 insertions(+), 3 deletions(-)
 create mode 100644 src/gcc-mainline/gcc/testsuite/gcc.dg/pr50193-1.c
 create mode 100644 src/gcc-mainline/gcc/testsuite/gcc.target/arm/shiftable.c

diff --git a/src/gcc-mainline/gcc/config/arm/predicates.md b/src/gcc-mainline/gcc/config/arm/predicates.md
index 27ba603..7307fd5 100644
--- a/src/gcc-mainline/gcc/config/arm/predicates.md
+++ b/src/gcc-mainline/gcc/config/arm/predicates.md
@@ -129,11 +129,12 @@
   (ior (match_operand 0 "arm_rhs_operand")
        (match_operand 0 "memory_operand")))
 
+;; This doesn't have to do much because the constant is already checked
+;; in the shift_operator predicate.
 (define_predicate "shift_amount_operand"
   (ior (and (match_test "TARGET_ARM")
 	    (match_operand 0 "s_register_operand"))
-       (and (match_code "const_int")
-	    (match_test "((unsigned HOST_WIDE_INT) INTVAL (op)) < 32"))))
+       (match_operand 0 "const_int_operand")))
 
 (define_predicate "arm_add_operand"
   (ior (match_operand 0 "arm_rhs_operand")
@@ -219,13 +220,21 @@
        (match_test "mode == GET_MODE (op)")))
 
 ;; True for shift operators.
+;; Notes:
+;;  * mult is only permitted with a constant shift amount
+;;  * patterns that permit register shift amounts only in ARM mode use
+;;    shift_amount_operand, patterns that always allow registers do not,
+;;    so we don't have to worry about that sort of thing here.
 (define_special_predicate "shift_operator"
   (and (ior (ior (and (match_code "mult")
 		      (match_test "power_of_two_operand (XEXP (op, 1), mode)"))
 		 (and (match_code "rotate")
 		      (match_test "GET_CODE (XEXP (op, 1)) == CONST_INT
 				   && ((unsigned HOST_WIDE_INT) INTVAL (XEXP (op, 1))) < 32")))
-	    (match_code "ashift,ashiftrt,lshiftrt,rotatert"))
+	    (and (match_code "ashift,ashiftrt,lshiftrt,rotatert")
+		 (ior (match_test "GET_CODE (XEXP (op, 1)) == CONST_INT
+				   && ((unsigned HOST_WIDE_INT) INTVAL (XEXP (op, 1))) < 32")
+		      (match_test "REG_P (XEXP (op, 1))"))))
        (match_test "mode == GET_MODE (op)")))
 
 ;; True for shift operators which can be used with saturation instructions.
diff --git a/src/gcc-mainline/gcc/testsuite/gcc.dg/pr50193-1.c b/src/gcc-mainline/gcc/testsuite/gcc.dg/pr50193-1.c
new file mode 100644
index 0000000..6abc9c4
--- /dev/null
+++ b/src/gcc-mainline/gcc/testsuite/gcc.dg/pr50193-1.c
@@ -0,0 +1,10 @@
+/* PR 50193: ARM: ICE on a | (b << negative-constant) */
+/* Ensure that the compiler doesn't ICE.  */
+
+/* { dg-options "-O2" } */
+
+int
+foo(int a, int b)
+{
+  return a | (b << -3); /* { dg-warning "left shift count is negative" } */
+}
diff --git a/src/gcc-mainline/gcc/testsuite/gcc.target/arm/shiftable.c b/src/gcc-mainline/gcc/testsuite/gcc.target/arm/shiftable.c
new file mode 100644
index 0000000..2d72bcc
--- /dev/null
+++ b/src/gcc-mainline/gcc/testsuite/gcc.target/arm/shiftable.c
@@ -0,0 +1,43 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+/* { dg-require-effective-target arm32 } */
+
+int
+plus (int a, int b)
+{
+  return (a * 64) + b;
+}
+
+/* { dg-final { scan-assembler "add.*\[al]sl #6" } } */
+
+int
+minus (int a, int b)
+{
+  return a - (b * 64);
+}
+
+/* { dg-final { scan-assembler "sub.*\[al]sl #6" } } */
+
+int
+ior (int a, int b)
+{
+  return (a * 64) | b;
+}
+
+/* { dg-final { scan-assembler "orr.*\[al]sl #6" } } */
+
+int
+xor (int a, int b)
+{
+  return (a * 64) ^ b;
+}
+
+/* { dg-final { scan-assembler "eor.*\[al]sl #6" } } */
+
+int
+and (int a, int b)
+{
+  return (a * 64) & b;
+}
+
+/* { dg-final { scan-assembler "and.*\[al]sl #6" } } */

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