gas: scfi: bugfixes for SCFI state propagation

There are two state propagation functions in SCFI machinery - forward
and backward flow.  The patch addresses two issues:
  - In forward_flow_scfi_state (), the state being compared in forward flow
    must be that at the exit of a prev bb and that at the entry of the
    next bb.  The variable holding the state to be compared was
    previously (erroneously) stale.
  - In cmp_scfi_state (), the assumption that two different control
    flows, leading to the same basic block, cannot have a mismatched
    notion of CFA base register, is not true.  Remove the assertion and
    instead return err if mismatch.

Fixing these issues helps correctly synthesize CFI, when previously
SCFI was erroring out for an otherwise valid input asm.

gas/
	* scfi.c (cmp_scfi_state): Remove assertion and return mismatch
	in return value as applicable.
	(forward_flow_scfi_state): Update state object to be the same as
	the exit state of the prev bb before comparing.

gas/testsuite/
	* gas/scfi/x86_64/scfi-x86-64.exp: Add new test.
	* gas/scfi/x86_64/scfi-cfg-5.d: New test.
	* gas/scfi/x86_64/scfi-cfg-5.l: New test.
	* gas/scfi/x86_64/scfi-cfg-5.s: New test.

gas/scfi.c

@@ -155,7 +155,7 @@ cmp_scfi_state (scfi_stateS *state1, scfi_stateS *state2)
   int ret;
 
   if (!state1 || !state2)
-    ret = 1;
+    return 1;
 
   /* Skip comparing the scratch[] value of registers.  The user visible
      unwind information is derived from the regs[] from the SCFI state.  */
@@ -164,10 +164,12 @@ cmp_scfi_state (scfi_stateS *state1, scfi_stateS *state2)
 
   /* For user functions which perform dynamic stack allocation, after switching
      t REG_FP based CFA tracking, it is perfectly possible to have stack usage
-     in some control flows.  However, double-checking that all control flows
-     have the same idea of CFA tracking before this wont hurt.  */
-  gas_assert (state1->regs[REG_CFA].base == state2->regs[REG_CFA].base);
-  if (state1->regs[REG_CFA].base == REG_SP)
+     in some control flows.  Further, the different control flows may even not
+     have the same idea of CFA tracking (likely later necessitating generation
+     of .cfi_remember_state / .cfi_restore_state pair).  */
+  ret |= state1->regs[REG_CFA].base != state2->regs[REG_CFA].base;
+
+  if (!ret && state1->regs[REG_CFA].base == REG_SP)
     ret |= state1->stack_size != state2->stack_size;
 
   ret |= state1->traceable_p != state2->traceable_p;
@@ -911,11 +913,11 @@ gen_scfi_ops (ginsnS *ginsn, scfi_stateS *state)
   return ret;
 }
 
-/* Recursively perform forward flow of the (unwind information) SCFI state
+/* Recursively perform forward flow of the (unwind information) SCFI STATE
    starting at basic block GBB.
 
-   The forward flow process propagates the SCFI state at exit of a basic block
-   to the successor basic block.
+   The core of forward flow process takes the SCFI state at the entry of a bb
+   and updates it incrementally as per the semantics of each ginsn in the bb.
 
    Returns error code, if any.  */
 
@@ -961,6 +963,8 @@ forward_flow_scfi_state (gcfgS *gcfg, gbbS *gbb, scfi_stateS *state)
       bb_for_each_edge(gbb, gedge)
 	{
 	  gbb = gedge->dst_bb;
+	  /* Ensure that the state is the one from the exit of the prev bb.  */
+	  memcpy (state, prev_bb->exit_state, sizeof (scfi_stateS));
 	  if (gbb->visited)
 	    {
 	      ret = cmp_scfi_state (gbb->entry_state, state);

gas/testsuite/gas/scfi/x86_64/scfi-cfg-5.d

@@ -0,0 +1,39 @@
+#as: --scfi=experimental -W
+#as:
+#objdump: -Wf
+#name: Synthesize CFI in presence of control flow 5
+#...
+Contents of the .eh_frame section:
+
+00000000 0+0014 0+0000 CIE
+  Version:               1
+  Augmentation:          "zR"
+  Code alignment factor: 1
+  Data alignment factor: -8
+  Return address column: 16
+  Augmentation data:     1b
+  DW_CFA_def_cfa: r7 \(rsp\) ofs 8
+  DW_CFA_offset: r16 \(rip\) at cfa-8
+  DW_CFA_nop
+  DW_CFA_nop
+
+0+0018 0+002c 0000001c FDE cie=00000000 pc=0+0000..0+0017
+  DW_CFA_advance_loc: 1 to 0+0001
+  DW_CFA_def_cfa_offset: 16
+  DW_CFA_offset: r6 \(rbp\) at cfa-16
+  DW_CFA_advance_loc: 3 to 0+0004
+  DW_CFA_def_cfa_register: r6 \(rbp\)
+  DW_CFA_advance_loc: 5 to 0+0009
+  DW_CFA_remember_state
+  DW_CFA_advance_loc: 6 to 0+000f
+  DW_CFA_def_cfa_register: r7 \(rsp\)
+  DW_CFA_restore: r6 \(rbp\)
+  DW_CFA_def_cfa_offset: 8
+  DW_CFA_advance_loc: 1 to 0+0010
+  DW_CFA_restore_state
+  DW_CFA_advance_loc: 6 to 0+0016
+  DW_CFA_def_cfa_register: r7 \(rsp\)
+  DW_CFA_restore: r6 \(rbp\)
+  DW_CFA_def_cfa_offset: 8
+
+#pass

gas/testsuite/gas/scfi/x86_64/scfi-cfg-5.l

@@ -0,0 +1,2 @@
+.*Assembler messages:
+.*5: Warning: SCFI ignores most user-specified CFI directives

gas/testsuite/gas/scfi/x86_64/scfi-cfg-5.s

@@ -0,0 +1,32 @@
+	.text
+	.globl foo
+	.type foo, @function
+foo:
+	.cfi_startproc
+	push  %rbp
+	.cfi_def_cfa_offset 16
+	.cfi_offset %rbp, -16
+	mov   %rsp, %rbp
+	.cfi_def_cfa_register %rbp
+	cmpl  $-1, %eax
+	jne .L1
+.L2:
+	.cfi_remember_state
+	call bar
+	pop  %rbp
+	.cfi_def_cfa_register %rsp
+	.cfi_restore %rbp
+	.cfi_def_cfa_offset 8
+	ret
+
+.L1:
+	.cfi_restore_state
+	testq %rax, %rax
+	je .L2
+	pop %rbp
+	.cfi_def_cfa_register %rsp
+	.cfi_restore %rbp
+	.cfi_def_cfa_offset 8
+	ret
+	.cfi_endproc
+	.size foo,.-foo

gas/testsuite/gas/scfi/x86_64/scfi-x86-64.exp

@@ -82,6 +82,8 @@ if  { ([istarget "x86_64-*-*"] && ![istarget "x86_64-*-linux*-gnux32"]) } then {
     run_list_test "scfi-cfg-3" "--scfi=experimental --warn"
     run_dump_test "scfi-cfg-4"
     run_list_test "scfi-cfg-4" "--scfi=experimental --warn"
+    run_dump_test "scfi-cfg-5"
+    run_list_test "scfi-cfg-5" "--scfi=experimental --warn"
     run_dump_test "scfi-asm-marker-1"
     run_list_test "scfi-asm-marker-1" "--scfi=experimental --warn"
     run_dump_test "scfi-asm-marker-2"