Patchwork [AArch64] fix missing Dwarf call frame information in the epilogue

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Submitter Yufeng Zhang
Date Sept. 12, 2012, 4:10 p.m.
Message ID <5050B3F0.4090702@arm.com>
Download mbox | patch
Permalink /patch/183403/
State New
Headers show

Comments

Yufeng Zhang - Sept. 12, 2012, 4:10 p.m.
This patch fixes a bug in the AArch64 epilogue expander which failed to 
generate consistent information of REG_FRAME_RELATED_EXPR reg notes in 
some cases; for instance, given the following C source code:

int bar (unsigned int);

int foo (void)
{
    return bar (0xcafe);
}
------------------- CUT -------------------

The -O0 -g code gen for the function 'foo' is:

          .cfi_startproc
          stp     x29, x30, [sp, -16]!
.LCFI0:
          .cfi_def_cfa_offset 16
          .cfi_offset 29, -16
          .cfi_offset 30, -8
          add     x29, sp, 0
.LCFI1:
          .cfi_def_cfa_register 29
.LM2:
          mov     w0, 57005
          bl      bar
.LM3:
          ldp     x29, x30, [sp], 16
          .cfi_restore 30
          .cfi_restore 29
          ret
          .cfi_endproc
------------------- CUT -------------------

As can be seen, the CFA reg is X29 from the ADD instruction until the
end of the func; this is wrong as the LDP instruction restores X29;
the CFA reg should be set to SP after LDP.

The patch passes the gcc and g++ testsuites.  Is it OK for the aarch64 
branch?


Thanks,
Yufeng


gcc/ChangeLog

2012-09-12  Yufeng Zhang  <yufeng.zhang@arm.com>

         * config/aarch64/aarch64.c (aarch64_expand_prologue): Add new local
         variable 'cfa_reg' and replace stack_pointer_rtx with it in the
         REG_FRAME_RELATED_EXPR reg note for the load-pair with writeback
         instruction.

gcc/testsuite/ChangeLog

2012-09-12  Yufeng Zhang  <yufeng.zhang@arm.com>

         * gcc.target/aarch64/dwarf-cfa-reg.c: New file.
Richard Henderson - Sept. 12, 2012, 6:37 p.m.
On 09/12/2012 09:10 AM, Yufeng Zhang wrote:
>  	      aarch64_set_frame_expr (gen_rtx_SET
>  				      (Pmode,
>  				       stack_pointer_rtx,
> -				       gen_rtx_PLUS (Pmode, stack_pointer_rtx,
> +				       gen_rtx_PLUS (Pmode, cfa_reg,
>  						     GEN_INT (offset))));

We'd prefer to use

  plus_constant (Pmode, cfa_reg, offset)

instead of the explicit call to gen_rtx_PLUS and GEN_INT.
It would appear that the entire aarch64.c file ought to 
be audited for that.

Also, use of the REG_CFA_* notes is strongly encouraged over
use of REG_FRAME_RELATED_EXPR.

There's all sorts of work involved in turning R_F_R_E into
R_CFA_* notes, depending on a rather large state machine.
This state machine was developed when only prologues were
annotated for unwinding, and therefore one cannot expect it
to work reliably for epilogues.

A long-term goal is to convert all targets to use R_CFA_*
exclusively, as that preserves much more information present
in the structure of the code of the prologue generator.  It
means less work within the compiler, and eventually being able
to remove a rather large hunk of state-machine code.


r~

Patch

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 2d7eba7..f452d3d 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -1786,7 +1786,7 @@  aarch64_expand_prologue (void)
 	       - original_frame_size
 	       - cfun->machine->frame.saved_regs_size);
 
-  /* Store pairs and load pairs have a range only of +/- 512.  */
+  /* Store pairs and load pairs have a range only -512 to 504.  */
   if (offset >= 512)
     {
       /* When the frame has a large size, an initial decrease is done on
@@ -1934,6 +1934,7 @@  aarch64_expand_epilogue (bool for_sibcall)
   HOST_WIDE_INT original_frame_size, frame_size, offset;
   HOST_WIDE_INT fp_offset;
   rtx insn;
+  rtx cfa_reg;
 
   aarch64_layout_frame ();
   original_frame_size = get_frame_size () + cfun->machine->saved_varargs_size;
@@ -1946,7 +1947,9 @@  aarch64_expand_epilogue (bool for_sibcall)
 	       - original_frame_size
 	       - cfun->machine->frame.saved_regs_size);
 
-  /* Store pairs and load pairs have a range only of +/- 512.  */
+  cfa_reg = frame_pointer_needed ? hard_frame_pointer_rtx : stack_pointer_rtx;
+
+  /* Store pairs and load pairs have a range only -512 to 504.  */
   if (offset >= 512)
     {
       offset = original_frame_size + cfun->machine->frame.saved_regs_size;
@@ -1977,6 +1980,10 @@  aarch64_expand_epilogue (bool for_sibcall)
 				       hard_frame_pointer_rtx,
 				       GEN_INT (- fp_offset)));
       RTX_FRAME_RELATED_P (insn) = 1;
+      /* As SP is set to (FP - fp_offset), according to the rules in
+	 dwarf2cfi.c:dwarf2out_frame_debug_expr, CFA should be calculated
+	 from the value of SP from now on.  */
+      cfa_reg = stack_pointer_rtx;
     }
 
   aarch64_save_or_restore_callee_save_registers
@@ -2016,10 +2023,15 @@  aarch64_expand_epilogue (bool for_sibcall)
 				 GEN_INT (offset),
 				 GEN_INT (GET_MODE_SIZE (DImode) + offset)));
 	      RTX_FRAME_RELATED_P (XVECEXP (PATTERN (insn), 0, 2)) = 1;
+	      /* The REG_FRAME_RELATED_EXPR to be attached to the above insn
+		 is either SP = SP + offset or SP = FP + offset, depending on
+		 which register had been used to calculate the CFA before
+		 the insn; note that FP had the same value as SP before the
+		 above insn.  */
 	      aarch64_set_frame_expr (gen_rtx_SET
 				      (Pmode,
 				       stack_pointer_rtx,
-				       gen_rtx_PLUS (Pmode, stack_pointer_rtx,
+				       gen_rtx_PLUS (Pmode, cfa_reg,
 						     GEN_INT (offset))));
 	    }
 
@@ -2040,7 +2052,6 @@  aarch64_expand_epilogue (bool for_sibcall)
 	      RTX_FRAME_RELATED_P (insn) = 1;
 	    }
 	}
-
       else
 	{
 	  insn = emit_insn (gen_add2_insn (stack_pointer_rtx,
diff --git a/gcc/testsuite/gcc.target/aarch64/dwarf-cfa-reg.c b/gcc/testsuite/gcc.target/aarch64/dwarf-cfa-reg.c
new file mode 100644
index 0000000..cce8815
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/dwarf-cfa-reg.c
@@ -0,0 +1,14 @@ 
+/* Verify that CFA register is restored to SP after FP is restored.  */
+/* { dg-do compile } */
+/* { dg-options "-O0 -gdwarf-2" } */
+/* { dg-final { scan-assembler ".cfi_restore 30" } } */
+/* { dg-final { scan-assembler ".cfi_restore 29" } } */
+/* { dg-final { scan-assembler ".cfi_def_cfa 31, 0" } } */
+/* { dg-final { scan-assembler "ret" } } */
+
+int bar (unsigned int);
+
+int foo (void)
+{
+  return bar (0xcafe);
+}