[PATCH v4 13/15] gas: Don't skip SFrame FDE if .cfi_register specifies SP register

[Jens Remus]

Neither ".cfi_offset SP, <offset>", ".cfi_register SP, <regno>", nor
".cfi_val_offset SP, <offset>" alter the tracking information to recover
the stack pointer (SP). Doing so would need an explicit .cfi_def_cfa,
which SFrame tracks.

The stack-pointer (SP) register contents on entry can be reconstructed
from the SFrame CFA tracking information using information from the
current and initial SFrame FREs of the SFrame FDE:

1. Compute CFA from the current CFA base register (SP or FP) and CFA
   offset from the SFrame CFA tracking information from the SFrame FRE
   for the current instruction address:

     CFA = <current_base_reg> + <current_cfa_offset>

2. Compute SP from the current CFA and the CFA offset from the SFrame
   CFA tracking information from the initial SFrame FRE of the FDE:

     SP = CFA - <initial_cfa_offset>

While at it add comments to the processing of .cfi_offset and
.cfi_val_offset that the SP can be reconstructed from the CFA tracking
information.

gas/
	* gen-sframe.c (sframe_xlate_do_register): Do not skip SFrame
	FDE if .cfi_register specifies SP register.
	(sframe_xlate_do_offset,sframe_xlate_do_val_offset): Add comment
	that this is likewise.

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

Notes (jremus):
    Changes v3 -> v4:
    - Add comment to sframe_xlate_do_offset why SP register is ignored.
    - Reword commit message accordingly.
    
    Changes v2 -> v3:
    - New patch.

 gas/gen-sframe.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/gas/gen-sframe.c b/gas/gen-sframe.c
index 31f2e5118280..f4bdbb5944d9 100644
--- a/gas/gen-sframe.c
+++ b/gas/gen-sframe.c
@@ -1100,6 +1100,7 @@ sframe_xlate_do_offset (struct sframe_xlate_ctx *xlate_ctx,
   gas_assert (cur_fre);
   /* Change the rule for the register indicated by the register number to
      be the specified offset.  */
+  /* Ignore SP reg, as it can be recovered from the CFA tracking info.  */
   if (cfi_insn->u.r == SFRAME_CFA_FP_REG)
     {
       gas_assert (!cur_fre->base_reg);
@@ -1134,6 +1135,7 @@ sframe_xlate_do_val_offset (struct sframe_xlate_ctx *xlate_ctx ATTRIBUTE_UNUSED,
 #ifdef SFRAME_FRE_RA_TRACKING
       || (sframe_ra_tracking_p () && cfi_insn->u.r == SFRAME_CFA_RA_REG)
 #endif
+      /* Ignore SP reg, as it can be recovered from the CFA tracking info.  */
       )
     {
       as_warn (_("skipping SFrame FDE; %s register %u in .cfi_val_offset"),
@@ -1153,14 +1155,15 @@ sframe_xlate_do_register (struct sframe_xlate_ctx *xlate_ctx ATTRIBUTE_UNUSED,
 			  struct cfi_insn_data *cfi_insn)
 {
   /* Previous value of register1 is register2.  However, if the specified
-     register1 is not interesting (SP, FP, or RA reg), the current DW_CFA_register
+     register1 is not interesting (FP or RA reg), the current DW_CFA_register
      instruction can be safely skipped without sacrificing the asynchronicity of
      stack trace information.  */
-  if (cfi_insn->u.rr.reg1 == SFRAME_CFA_SP_REG
+  if (cfi_insn->u.rr.reg1 == SFRAME_CFA_FP_REG
 #ifdef SFRAME_FRE_RA_TRACKING
       || (sframe_ra_tracking_p () && cfi_insn->u.rr.reg1 == SFRAME_CFA_RA_REG)
 #endif
-      || cfi_insn->u.rr.reg1 == SFRAME_CFA_FP_REG)
+      /* Ignore SP reg, as it can be recovered from the CFA tracking info.  */
+      )
     {
       as_warn (_("skipping SFrame FDE; %s register %u in .cfi_register"),
 	       sframe_register_name (cfi_insn->u.rr.reg1), cfi_insn->u.rr.reg1);
-- 
2.40.1