[PATCH, V3 1/7] gas: scfi: make scfi_state_restore_reg function more precise

[Indu Bhagat]

[No changes in V3]
[New in V2]
 - This patch is orthogonal to aarch64 support.  It is included here
   because the testcase added for it is aarch64 specific.  It is a
   bugfix really.
[End of New in V2]

When the SCFI machinery detects that a register has been restored from
stack, it makes some state changes in the SCFI state object.

Prior to the patch, scfi_state_restore_reg () was setting a value of
(reg, CFI_IN_REG) for (base, state) respectively.  This was causing
issues in the cmp_scfi_state () function:
  - The default state of all (callee-saved) regs at the beginning of
    function is set to (0, CFI_UNDEFINED).
  - If a register is saved and restored on some control path, the state
    of reg is (reg, CFI_IN_REG) on that path.
  - On another control path where the register was perhaps not
    used (or saved/restored on stack) remains (0, CFI_UNDEFINED).
  - The two states should be treated equal, however, at the point in
    program after the register has been restored.

Fix this by resetting the state to (0, CFI_UNDEFINED) in
scfi_state_restore_reg ().

A testcase (scfi-cfg-4.s) for this is added in a subsequent commit.

gas/
        * scfi.c (scfi_state_restore_reg): Reset to 0, CFI_UNDEFINED
	for base, state.
---
Notes:
 - The testcase for this patch will be added with the SCFI/aarch64
   patches.
 - This patch needs a global maintainer's review/approval.

 gas/scfi.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/gas/scfi.c b/gas/scfi.c
index 744822d8102..5898a57b330 100644
--- a/gas/scfi.c
+++ b/gas/scfi.c
@@ -223,11 +223,12 @@ scfi_state_restore_reg (scfi_stateS *state, unsigned int reg)
   gas_assert (state->regs[reg].state == CFI_ON_STACK);
   gas_assert (state->regs[reg].base == REG_CFA);
 
-  state->regs[reg].base = reg;
+  /* PS: the register may still be on stack much after the restore.  Reset the
+     SCFI state to CFI_UNDEFINED, however, to indicate that the most updated
+     source of value is register itself from here onwards.  */
+  state->regs[reg].base = 0;
   state->regs[reg].offset = 0;
-  /* PS: the register may still be on stack much after the restore, but the
-     SCFI state keeps the state as 'in register'.  */
-  state->regs[reg].state = CFI_IN_REG;
+  state->regs[reg].state = CFI_UNDEFINED;
 }
 
 /* Identify if the given GAS instruction GINSN saves a register
-- 
2.43.0