[PATCH] rs6000: Don't touch below the stack pointer (PR77687)

[Segher Boessenkool]

With the 32-bit SVR4 ABI we don't have a red zone, so we have to restore
the callee-saved registers before we restore the stack pointer.

The previous fix for this PR failed in two ways, for huge frames: first,
we use a negative offset from r11 in that case, so the (mem:BLK 11) access
does no good; second, sched does not handle accesses to mem:BLK correctly
in this case (does not make dependencies).

This patch fixes it by doing a store to (mem:BLK (scratch)) instead.
This means no unrelated (not to stack) loads/stores can be moved over the
stack restore either, but so be it.

I intend to commit this tomorrow if there are no objections.


Segher


2017-09-19  Segher Boessenkool  <seg...@kernel.crashing.org>

        PR target/77687
        * config/rs6000/rs6000.md (stack_restore_tie): Store to a scratch
        address instead of to r1 and r11.

gcc/testsuite/
        PR target/77687
        * gcc.target/powerpc/pr77687.c: New testcase.


---
 gcc/config/rs6000/rs6000.md                |  6 ++----
 gcc/testsuite/gcc.target/powerpc/pr77687.c | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/powerpc/pr77687.c

diff --git a/gcc/config/rs6000/rs6000.md b/gcc/config/rs6000/rs6000.md
index 7f17628..13ba743 100644
--- a/gcc/config/rs6000/rs6000.md
+++ b/gcc/config/rs6000/rs6000.md
@@ -13161,14 +13161,12 @@ (define_insn "stack_tie"
 
 ; Some 32-bit ABIs do not have a red zone, so the stack deallocation has to
 ; stay behind all restores from the stack, it cannot be reordered to before
-; one.  See PR77687.  This insn is an add or mr, and a stack_tie on the
-; operands of that.
+; one.  See PR77687.  This insn is an add or mr, and a memory clobber.
 (define_insn "stack_restore_tie"
   [(set (match_operand:SI 0 "gpc_reg_operand" "=r,r")
        (plus:SI (match_operand:SI 1 "gpc_reg_operand" "r,r")
                 (match_operand:SI 2 "reg_or_cint_operand" "O,rI")))
-   (set (mem:BLK (match_dup 0)) (const_int 0))
-   (set (mem:BLK (match_dup 1)) (const_int 0))]
+   (set (mem:BLK (scratch)) (const_int 0))]
   "TARGET_32BIT"
   "@
    mr %0,%1
diff --git a/gcc/testsuite/gcc.target/powerpc/pr77687.c 
b/gcc/testsuite/gcc.target/powerpc/pr77687.c
new file mode 100644
index 0000000..95b27ae
--- /dev/null
+++ b/gcc/testsuite/gcc.target/powerpc/pr77687.c
@@ -0,0 +1,20 @@
+/* { dg-options "-std=gnu99 -O2" } */
+/* { dg-final { scan-assembler-not {\mmr r?1,r?11\M.*11.*\mblr\M} } } */
+
+/* PR77687: We used to do stack accesses (via r11) after restoring r1.  */
+
+void g(int, char *);
+const char * dum = "hello";
+
+void f(int x)
+{
+       char big[200000];
+ start:
+       g(x, big);
+       g(x, big);
+       register void *p asm("r11") = &&start;
+       asm("" : : "r"(p));
+       asm("" : : :"r28");
+       asm("" : : :"r29");
+       asm("" : : :"r30");
+}
-- 
1.8.3.1