gas: Don't skip SFrame FDE if .cfi_register specifies SP register

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

1. Compute CFA from the current CFA base register (SP or FP) and CFA
   offset from the SFrame CFA base register 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 a comment to the processing of .cfi_val_offset that the
SP can be reconstructed from the CFA base register tracking information.

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

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

1 files changed, 5 insertions, 3 deletions

diff --git a/gas/gen-sframe.c b/gas/gen-sframe.c
index 1da7a77..20ba9e0 100644
--- a/gas/gen-sframe.c
+++ b/gas/gen-sframe.c
@@ -1138,6 +1138,7 @@ 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 due to .cfi_val_offset specifying %s register"),
@@ -1157,14 +1158,15 @@ 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 due to .cfi_register specifying %s register"),
 	       sframe_register_name (cfi_insn->u.rr.reg1));