[binutils-gdb] gas: scfi: make scfi_state_restore_reg function more precise

[Indu Bhagat]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=af4eb9e20a6b4d9a5e61d9caaf89daf4c3742150

commit af4eb9e20a6b4d9a5e61d9caaf89daf4c3742150
Author: Indu Bhagat <indu.bhagat@oracle.com>
Date:   Thu Jul 18 20:24:00 2024 -0700

    gas: scfi: make scfi_state_restore_reg function more precise
    
    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.

Diff:
---
 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