RFA: Tighten checking for 'X' constraints

[Richard Sandiford]

As Robert pointed out here:

    http://gcc.gnu.org/ml/gcc-patches/2014-04/msg00416.html

we're a bit too eager when folding stuff into an 'X' constraint.
The value at expand time is sensible, but after that asm_operand_ok
allows arbitrary rtx expressions, including any number of registers
as well as MEMs with unchecked addresses.

This is a target-independent problem, as shown by the testcase below.
Reload would give bogus "impossible constraint in asm" errors
while LRA ICEs.

Tested on x86_64-linux-gnu.  OK to install?

Thanks,
Richard


gcc/
        * recog.c (asm_operand_ok): Tighten MEM validity for 'X'.

gcc/testsuite/
        * gcc.dg/torture/asm-x-constraint-1.c: New test.

Index: gcc/recog.c
===================================================================
--- gcc/recog.c 2014-04-12 22:43:54.729854903 +0100
+++ gcc/recog.c 2014-04-15 21:47:32.139873570 +0100
@@ -1840,7 +1840,17 @@ asm_operand_ok (rtx op, const char *cons
          break;
 
        case 'X':
-         result = 1;
+         /* Although the asm itself doesn't impose any restrictions on
+            the operand, we still need to restrict it to something that
+            can be reloaded and printed.
+
+            MEM operands are always reloaded to make them legitimate,
+            regardless of the constraint, so we need to handle them
+            in the same way as for 'm' and 'g'.  Since 'X' is not treated
+            as an address constraint, the only other valid operand types
+            are constants and registers.  */
+         result = (CONSTANT_P (op)
+                   || general_operand (op, VOIDmode));
          break;
 
        case 'g':
Index: gcc/testsuite/gcc.dg/torture/asm-x-constraint-1.c
===================================================================
--- /dev/null   2014-04-15 08:10:27.294524132 +0100
+++ gcc/testsuite/gcc.dg/torture/asm-x-constraint-1.c   2014-04-15 
19:11:29.830962008 +0100
@@ -0,0 +1,27 @@
+void
+noprop1 (int **x, int y, int z)
+{
+  int *ptr = *x + y * z / 11;
+  __asm__ __volatile__ ("noprop1 %0" : : "X" (*ptr));
+}
+
+void
+noprop2 (int **x, int y, int z)
+{
+  int *ptr = *x + y * z / 11;
+  __asm__ __volatile__ ("noprop2 %0" : : "X" (ptr));
+}
+
+int *global_var;
+
+void
+const1 (void)
+{
+  __asm__ __volatile__ ("const1 %0" : : "X" (global_var));
+}
+
+void
+const2 (void)
+{
+  __asm__ __volatile__ ("const2 %0" : : "X" (*global_var));
+}