[RFC PATCH v3 2/2] s390: SFrame track FP/RA saved in register

[Jens Remus <jremus@linux.ibm.com>]

GCC on s390x, when in a leaf function, can be observed to save the
frame pointer (FP) and/or return address (RA) register in a floating-
point registers (FPR) instead of on the stack. This is declared using
the following CFI directive:

  .cfi_register <fp/ra-regno>, <fpr-regno>

SFrame cannot represent the FP and/or RA being saved in another
register. It does only track the CFA base register (SP/FP), CFA offset
from CFA base register, and FP and RA save area offsets from CFA.

On s390x the FP and/or RA are only saved in another FPR when in a leaf
function. That is a function that does not call any other functions.
Therefore it can ever only be the topmost function in a call chain.
During function return, if the RA would be restored into a non-default
RA register, the function would also only ever be the topmost function
on the call stack.
An unwinder by default has access to all registers of the function that
if the topmost on the call stack. Therefore no further information would
be required for those registers.

Represent the FP/RA in another register on s390, by encoding the
register number shifted by one to the left with the least-significant
bit set in the offset as follows:

  offset = (regno << 1) | 1

Add s390-specific SFrame (error) test cases for FP/RA being saved in
FPRs in leaf-function.

gas/
	* gen-sframe.c (s390_sframe_xlate_do_register): New s390-
	specific function to represent FP/RA in another register on
	s390.
	(sframe_xlate_do_register): Invoke s390_sframe_xlate_do_register
	on s390.

libsframe/
	* sframe-dump.c (is_sframe_abi_arch_s390): New helper to test
	whether ABI/arch is s390.
	(dump_sframe_func_with_fres): Dump FP/RA in another register on
	s390.

gas/testsuite/
	* gas/cfi-sframe/cfi-sframe.exp: Update s390-specific SFrame
	(error) test cases.
	* gas/cfi-sframe/cfi-sframe-s390-3.s: New s390-specific SFrame
	test case for FP/RA saved in register on s390.
	* gas/cfi-sframe/cfi-sframe-s390-3.d: Likewise.
	* gas/cfi-sframe/cfi-sframe-s390-err-6.s: New s390-specific
	SFrame error test case for FP without RA saved in register.
	* gas/cfi-sframe/cfi-sframe-s390-err-6.d: Likewise.
        * gas/cfi-sframe/cfi-sframe-s390-err-3.s: s390-specific SFrame
	error test case 3 no longer triggers an error and is renamed as
	test case 4.
        * gas/cfi-sframe/cfi-sframe-s390-err-3.d: Likewise.
        * gas/cfi-sframe/cfi-sframe-s390-4.s: Likewise.
        * gas/cfi-sframe/cfi-sframe-s390-4.d: Likewise. Added expected
        SFrame dump output.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>

* gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-4.d: # GCL: missing file
* gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-3.d -> gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-6.d: # GCL: missing file
* gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-3.s -> gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-4.s: # GCL: missing file
---

Notes (jremus):
    Changes v2 -> v3:
    - New patch.
    
    Currently there is no mean to determine whether a function is a leaf
    function. Neither an annotation that a section of the assembler code
    does not call any function. Both would guarantee that the code will
    only ever appear topmost on the call stack, so that the unwinder by
    default has access to all registers.
    Therefore SFrame generation cannot determine whether saving of FP/RA
    registers in other registers such as FPR is actually valid. It needs
    to trust the CFI, as it would need to to anyway.
    
    The s390-specific functionality could be made dependent on TC_S390
    at compile-time instead of on sframe_get_abi_arch() at run-time.
    
    sframe_fre_set_{bp|ra}_track() could be renamed to
    sframe_fre_set_{fp|ra}_track_offset() and new
    sframe_fre_set_{fp|ra}_track_register() could be introduced. These
    would call some architecture-specific callback to perform the encoding
    of the dw2renum as offset.

 gas/gen-sframe.c                              | 35 +++++++++++++++++++
 .../gas/cfi-sframe/cfi-sframe-s390-3.d        | 22 ++++++++++++
 .../gas/cfi-sframe/cfi-sframe-s390-3.s        | 15 ++++++++
 .../gas/cfi-sframe/cfi-sframe-s390-4.d        | 21 +++++++++++
 ...frame-s390-err-3.s => cfi-sframe-s390-4.s} |  0
 ...e-s390-err-3.d => cfi-sframe-s390-err-6.d} |  2 +-
 .../gas/cfi-sframe/cfi-sframe-s390-err-6.s    | 15 ++++++++
 gas/testsuite/gas/cfi-sframe/cfi-sframe.exp   |  4 ++-
 libsframe/sframe-dump.c                       | 22 ++++++++++--
 9 files changed, 132 insertions(+), 4 deletions(-)
 create mode 100644 gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-3.d
 create mode 100644 gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-3.s
 create mode 100644 gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-4.d
 rename gas/testsuite/gas/cfi-sframe/{cfi-sframe-s390-err-3.s => cfi-sframe-s390-4.s} (100%)
 rename gas/testsuite/gas/cfi-sframe/{cfi-sframe-s390-err-3.d => cfi-sframe-s390-err-6.d} (74%)
 create mode 100644 gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-6.s

diff --git a/gas/gen-sframe.c b/gas/gen-sframe.c
index 93a5eebcc9d8..4cc86eb6c815 100644
--- a/gas/gen-sframe.c
+++ b/gas/gen-sframe.c
@@ -1150,6 +1150,37 @@ sframe_xlate_do_val_offset (struct sframe_xlate_ctx *xlate_ctx ATTRIBUTE_UNUSED,
   return SFRAME_XLATE_OK;
 }
 
+/* S390-specific translate DW_CFA_register into SFrame context.
+   Return SFRAME_XLATE_OK if success.  */
+
+static int
+s390_sframe_xlate_do_register (struct sframe_xlate_ctx *xlate_ctx,
+			       struct cfi_insn_data *cfi_insn)
+{
+  /* The scratchpad FRE currently being updated with each cfi_insn
+     being interpreted.  This FRE eventually gets linked in into the
+     list of FREs for the specific function.  */
+  struct sframe_row_entry *cur_fre = xlate_ctx->cur_fre;
+
+  gas_assert (cur_fre);
+
+  /* Change the rule for the register indicated by the register number to
+     be the specified register.  Encode the register number as offset by
+     shifting it to the left by one and setting the least-significant bit
+     (LSB).  The LSB can be used to differentiate offsets from register
+     numbers, as offsets from CFA are always a multiple of -8 on s390x.  */
+  if (cfi_insn->u.rr.reg1 == SFRAME_CFA_FP_REG)
+    sframe_fre_set_bp_track (cur_fre, cfi_insn->u.rr.reg2 << 1 | 1);
+#ifdef SFRAME_FRE_RA_TRACKING
+  else if (sframe_ra_tracking_p ()
+	   && cfi_insn->u.rr.reg1 == SFRAME_CFA_RA_REG)
+    sframe_fre_set_ra_track (cur_fre, cfi_insn->u.rr.reg2 << 1 | 1);
+#endif
+
+  /* Safe to skip.  */
+  return SFRAME_XLATE_OK;
+}
+
 /* Translate DW_CFA_register into SFrame context.
    Return SFRAME_XLATE_OK if success.  */
 
@@ -1157,6 +1188,10 @@ static int
 sframe_xlate_do_register (struct sframe_xlate_ctx *xlate_ctx ATTRIBUTE_UNUSED,
 			  struct cfi_insn_data *cfi_insn)
 {
+  /* Conditionally invoke S390-specific implementation.  */
+  if (sframe_get_abi_arch () == SFRAME_ABI_S390_ENDIAN_BIG)
+    return s390_sframe_xlate_do_register (xlate_ctx, cfi_insn);
+
   /* Previous value of register1 is register2.  However, if the specified
      register1 is not interesting (FP or RA reg), the current DW_CFA_register
      instruction can be safely skipped without sacrificing the asynchronicity of
diff --git a/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-3.d b/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-3.d
new file mode 100644
index 000000000000..722874113225
--- /dev/null
+++ b/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-3.d
@@ -0,0 +1,22 @@
+#objdump: --sframe=.sframe
+#name: SFrame generation on s390
+#...
+Contents of the SFrame section .sframe:
+
+  Header :
+
+    Version: SFRAME_VERSION_2
+    Flags: NONE
+    Num FDEs: 1
+    Num FREs: 5
+
+  Function Index :
+
+    func idx \[0\]: pc = 0x0, size = 26 bytes
+    STARTPC +CFA +FP +RA +
+    0+0000 +sp\+160 +u +u +
+    0+0004 +sp\+160 +u +r16 +
+    0+0008 +sp\+160 +r17 +r16 +
+    0+0014 +sp\+160 +u +r16 +
+    0+0018 +sp\+160 +u +u +
+#pass
diff --git a/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-3.s b/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-3.s
new file mode 100644
index 000000000000..1d4497178ada
--- /dev/null
+++ b/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-3.s
@@ -0,0 +1,15 @@
+	.cfi_sections .sframe
+	.cfi_startproc
+	ldgr	%f0,%r14
+	.cfi_register 14, 16
+	ldgr	%f2,%r11
+	.cfi_register 11, 17
+	la	%r11,0
+	la	%r14,0
+.Lreturn:
+	lgdr	%r11,%f2
+	.cfi_restore 11
+	lgdr	%r14,%f0
+	.cfi_restore 14
+	br	%r14
+	.cfi_endproc
diff --git a/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-4.d b/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-4.d
new file mode 100644
index 000000000000..3c013ac2b6fe
--- /dev/null
+++ b/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-4.d
@@ -0,0 +1,21 @@
+#name: SFrame generation on s390
+#as: --gsframe
+#objdump: --sframe=.sframe
+#...
+Contents of the SFrame section .sframe:
+
+  Header :
+
+    Version: SFRAME_VERSION_2
+    Flags: NONE
+    Num FDEs: 1
+    Num FREs: 2
+
+  Function Index :
+
+    func idx \[0\]: pc = 0x0, size = 6 bytes
+    STARTPC +CFA +FP +RA +
+    0+0000 +sp\+160 +u +u +
+    0+0004 +sp\+160 +u +r7 +
+
+#pass:
diff --git a/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-3.s b/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-4.s
similarity index 100%
rename from gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-3.s
rename to gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-4.s
diff --git a/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-3.d b/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-6.d
similarity index 74%
rename from gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-3.d
rename to gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-6.d
index 4b3262f02f02..57af27d26419 100644
--- a/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-3.d
+++ b/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-6.d
@@ -1,6 +1,6 @@
 #name: SFrame generation on s390
 #as: --gsframe
-#warning: skipping SFrame FDE due to .cfi_register specifying RA register
+#warning: skipping SFrame FDE due to FP without RA on stack
 #objdump: --sframe=.sframe
 #...
 Contents of the SFrame section .sframe:
diff --git a/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-6.s b/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-6.s
new file mode 100644
index 000000000000..48b01ac5f57b
--- /dev/null
+++ b/gas/testsuite/gas/cfi-sframe/cfi-sframe-s390-err-6.s
@@ -0,0 +1,15 @@
+	.cfi_sections .sframe
+	.cfi_startproc
+	ldgr	%f2,%r11
+	.cfi_register 11, 17
+	ldgr	%f0,%r14
+	.cfi_register 14, 16
+	la	%r11,0
+	la	%r14,0
+.Lreturn:
+	lgdr	%r14,%f0
+	.cfi_restore 14
+	lgdr	%r11,%f2
+	.cfi_restore 11
+	br	%r14
+	.cfi_endproc
diff --git a/gas/testsuite/gas/cfi-sframe/cfi-sframe.exp b/gas/testsuite/gas/cfi-sframe/cfi-sframe.exp
index f97fa9e170a2..8b97b02222b3 100644
--- a/gas/testsuite/gas/cfi-sframe/cfi-sframe.exp
+++ b/gas/testsuite/gas/cfi-sframe/cfi-sframe.exp
@@ -105,9 +105,11 @@ if { [istarget "aarch64*-*-*"] && [gas_sframe_check] } then {
 if { [istarget "s390x*-*-*"] && [gas_sframe_check] } then {
     run_dump_test "cfi-sframe-s390-1"
     run_dump_test "cfi-sframe-s390-2"
+    run_dump_test "cfi-sframe-s390-3"
+    run_dump_test "cfi-sframe-s390-4"
     run_dump_test "cfi-sframe-s390-err-1"
     run_dump_test "cfi-sframe-s390-err-2"
-    run_dump_test "cfi-sframe-s390-err-3"
     run_dump_test "cfi-sframe-s390-err-4"
     run_dump_test "cfi-sframe-s390-err-5"
+    run_dump_test "cfi-sframe-s390-err-6"
 }
diff --git a/libsframe/sframe-dump.c b/libsframe/sframe-dump.c
index 69633d53a33a..40ea531314ba 100644
--- a/libsframe/sframe-dump.c
+++ b/libsframe/sframe-dump.c
@@ -40,6 +40,14 @@ is_sframe_abi_arch_aarch64 (sframe_decoder_ctx *sfd_ctx)
   return aarch64_p;
 }
 
+/* Return TRUE if the SFrame section is associated with the s390 ABIs.  */
+
+static bool
+is_sframe_abi_arch_s390 (sframe_decoder_ctx *sfd_ctx)
+{
+  return sframe_decoder_get_abi_arch (sfd_ctx) == SFRAME_ABI_S390_ENDIAN_BIG;
+}
+
 static void
 dump_sframe_header (sframe_decoder_ctx *sfd_ctx)
 {
@@ -175,7 +183,12 @@ dump_sframe_func_with_fres (sframe_decoder_ctx *sfd_ctx,
 
       /* Dump SP/FP info.  */
       if (err[1] == 0)
-	sprintf (temp, "c%+d", fp_offset);
+	{
+	  if (is_sframe_abi_arch_s390 (sfd_ctx) && (fp_offset & 1))
+	    sprintf (temp, "r%d", fp_offset >> 1);
+	  else
+	    sprintf (temp, "c%+d", fp_offset);
+	}
       else
 	strcpy (temp, "u");
       printf ("%-10s", temp);
@@ -187,7 +200,12 @@ dump_sframe_func_with_fres (sframe_decoder_ctx *sfd_ctx,
 	  != SFRAME_CFA_FIXED_RA_INVALID)
 	strcpy (temp, "f");
       else if (err[2] == 0)
-	sprintf (temp, "c%+d", ra_offset);
+	{
+	  if (is_sframe_abi_arch_s390 (sfd_ctx) && (ra_offset & 1))
+	    sprintf (temp, "r%d", ra_offset >> 1);
+	  else
+	    sprintf (temp, "c%+d", ra_offset);
+	}
       else
 	strcpy (temp, "u");
 
-- 
2.40.1