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Re: [PATCH][AArch64] Simplify frame layout for stack probing


From: Wilco Dijkstra
Sent: 25 July 2017 14:58
To: GCC Patches; James Greenhalgh; Jeff Law
Cc: nd
Subject: [PATCH][AArch64] Simplify frame layout for stack probing
This patch makes some changes to the frame layout in order to simplify
stack probing.  We want to use the save of LR as a probe in any non-leaf
function.  With shrinkwrapping we may only save LR before a call, so it
is useful to define a fixed location in the callee-saves. So force LR at
the bottom of the callee-saves even with -fomit-frame-pointer.

Also remove a rarely used frame layout that saves the callee-saves first
with -fomit-frame-pointer.

OK for commit (and backport to GCC7)?

2017-07-25  Wilco Dijkstra  <>

        * config/aarch64/aarch64.c (aarch64_layout_frame):
        Ensure LR is always stored at the bottom of the callee-saves.
        Remove frame option which saves callee-saves at top of frame.

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index b8a4160d9de8e689ccd26cb9f0ce046ee65e0ef4..3fc36ae28d18b9635480fd99f1fa7719267e66e4 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -2875,7 +2875,8 @@ aarch64_frame_pointer_required (void)
 /* Mark the registers that need to be saved by the callee and calculate
    the size of the callee-saved registers area and frame record (both FP
-   and LR may be omitted).  */
+   and LR may be omitted).  If the function is not a leaf, ensure LR is
+   saved at the bottom of the callee-save area.  */
 static void
 aarch64_layout_frame (void)
@@ -2926,7 +2927,14 @@ aarch64_layout_frame (void)
       cfun->machine->frame.wb_candidate1 = R29_REGNUM;
       cfun->machine->frame.reg_offset[R30_REGNUM] = UNITS_PER_WORD;
       cfun->machine->frame.wb_candidate2 = R30_REGNUM;
-      offset += 2 * UNITS_PER_WORD;
+      offset = 2 * UNITS_PER_WORD;
+    }
+  else if (!crtl->is_leaf)
+    {
+      /* Ensure LR is saved at the bottom of the callee-saves.  */
+      cfun->machine->frame.reg_offset[R30_REGNUM] = 0;
+      cfun->machine->frame.wb_candidate1 = R30_REGNUM;
+      offset = UNITS_PER_WORD;
   /* Now assign stack slots for them.  */
@@ -3025,20 +3033,6 @@ aarch64_layout_frame (void)
         = cfun->machine->frame.frame_size - cfun->machine->frame.callee_adjust;
-  else if (!frame_pointer_needed
-          && varargs_and_saved_regs_size < max_push_offset)
-    {
-      /* Frame with large local area and outgoing arguments (this pushes the
-        callee-saves first, followed by the locals and outgoing area):
-        stp reg1, reg2, [sp, -varargs_and_saved_regs_size]!
-        stp reg3, reg4, [sp, 16]
-        sub sp, sp, frame_size - varargs_and_saved_regs_size  */
-      cfun->machine->frame.callee_adjust = varargs_and_saved_regs_size;
-      cfun->machine->frame.final_adjust
-       = cfun->machine->frame.frame_size - cfun->machine->frame.callee_adjust;
-      cfun->machine->frame.hard_fp_offset = cfun->machine->frame.callee_adjust;
-      cfun->machine->frame.locals_offset = cfun->machine->frame.hard_fp_offset;
-    }
       /* Frame with large local area and outgoing arguments using frame pointer:

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