Tighten use of HARD_FRAME_POINTER_REGNUM in alias.c (PR 84538)

RTL code needs to be consistent about whether it uses the stack
pointer, the frame pointer or the argument pointer to access a
given part of the frame.  alias.c used this to divide accesses
into three independent areas.

The problem in the PR is that we did this for HARD_FRAME_POINTER_REGNUM
even when the register wasn't being used as a frame pointer.  We can't
do that because the frame pointer is then just any old allocatable
register and could certainly point to info accessed through the
argument pointer or stack pointer.

2018-03-01  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR rtl-optimization/84538
	* alias.c (init_alias_target): Add commentary.
	(init_alias_analysis): Only give HARD_FRAME_POINTER_REGNUM
	a unique base value if the frame pointer is not eliminated
	to the stack pointer.

gcc/testsuite/
	PR rtl-optimization/84538
	* gcc.dg/torture/pr84538.c: New test.

From-SVN: r258094

4 files changed, 46 insertions, 1 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 0a07f4b..0d93e26 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,11 @@
+2018-03-01  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	PR rtl-optimization/84538
+	* alias.c (init_alias_target): Add commentary.
+	(init_alias_analysis): Only give HARD_FRAME_POINTER_REGNUM
+	a unique base value if the frame pointer is not eliminated
+	to the stack pointer.
+
 2018-03-01  Tom de Vries  <tom@codesourcery.com>
 
 	PR rtl-optimization/83327
diff --git a/gcc/alias.c b/gcc/alias.c
index 4c7965d..5fa15cb 100644
--- a/gcc/alias.c
+++ b/gcc/alias.c
@@ -3191,12 +3191,21 @@ init_alias_target (void)
 	&& targetm.hard_regno_mode_ok (i, Pmode))
       static_reg_base_value[i] = arg_base_value;
 
+  /* RTL code is required to be consistent about whether it uses the
+     stack pointer, the frame pointer or the argument pointer to
+     access a given area of the frame.  We can therefore use the
+     base address to distinguish between the different areas.  */
   static_reg_base_value[STACK_POINTER_REGNUM]
     = unique_base_value (UNIQUE_BASE_VALUE_SP);
   static_reg_base_value[ARG_POINTER_REGNUM]
     = unique_base_value (UNIQUE_BASE_VALUE_ARGP);
   static_reg_base_value[FRAME_POINTER_REGNUM]
     = unique_base_value (UNIQUE_BASE_VALUE_FP);
+
+  /* The above rules extend post-reload, with eliminations applying
+     consistently to each of the three pointers.  Cope with cases in
+     which the frame pointer is eliminated to the hard frame pointer
+     rather than the stack pointer.  */
   if (!HARD_FRAME_POINTER_IS_FRAME_POINTER)
     static_reg_base_value[HARD_FRAME_POINTER_REGNUM]
       = unique_base_value (UNIQUE_BASE_VALUE_HFP);
@@ -3329,7 +3338,14 @@ init_alias_analysis (void)
 
       /* Initialize the alias information for this pass.  */
       for (i = 0; i < FIRST_PSEUDO_REGISTER; i++)
-	if (static_reg_base_value[i])
+	if (static_reg_base_value[i]
+	    /* Don't treat the hard frame pointer as special if we
+	       eliminated the frame pointer to the stack pointer instead.  */
+	    && !(i == HARD_FRAME_POINTER_REGNUM
+		 && reload_completed
+		 && !frame_pointer_needed
+		 && targetm.can_eliminate (FRAME_POINTER_REGNUM,
+					   STACK_POINTER_REGNUM)))
 	  {
 	    new_reg_base_value[i] = static_reg_base_value[i];
 	    bitmap_set_bit (reg_seen, i);
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index aac4964..6528ac0 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2018-03-01  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	PR rtl-optimization/84538
+	* gcc.dg/torture/pr84538.c: New test.
+
 2018-02-28  Martin Sebor  <msebor@redhat.com>
 
 	* c-c++-common/Warray-bounds-2.c: Declare helper static to avoid
diff --git a/gcc/testsuite/gcc.dg/torture/pr84538.c b/gcc/testsuite/gcc.dg/torture/pr84538.c
new file mode 100644
index 0000000..b56a1c4
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr84538.c
@@ -0,0 +1,16 @@
+/* { dg-do run } */
+/* { dg-additional-options "-fno-omit-frame-pointer -w" } */
+
+#define SIZE 8
+
+main()
+{
+  int a[SIZE] = {1};
+  int i;
+
+  for (i = 1; i < SIZE; i++)
+    if (a[i] != 0)
+      abort();
+
+  exit (0);
+}