[PATCH] readelf: Remove show_op_index variable

[PATCH] readelf: Remove show_op_index variable

[tbaeder]

From: Timm Bäder <tbaeder@redhat.com>

advance_pc() uses show_op_index to save whether the current op_index is
> 0 OR the new op_index is > 0.

The new op index is calculated via

new_op_index = (op_index + op_advance) % max_ops_per_instr;

since all of the variables involved are unsigned,
new_op_index >= op_index is always true.

So...

if op_index > 0, then new_op_index > 0
if op_index == 0, then new_op_index >= 0

and if the new_op_index is > 0, then the old one was as well.

In any case, we only need to check the new_op_index, since show_op_index
used to OR the two comparisons.

In other words:

 op_index > 0  |  new_op_index > 0  || show_op_index
 ------------------------------------------------
      true           true                true
     false           true                true
      true          false                true     xx
     false          false                false

... but since the third line (marked with xx) is not possible,
the table becomes:

 op_index > 0  |  new_op_index > 0  || show_op_index
 ------------------------------------------------
      true           true                true
     false           true                true
     false          false                false

... and show_op_index is equal to (new_op_index > 0).
So, remove the show_op_index variable and simply replace it by comparing
the new op_index > 0.
---
 src/ChangeLog | 5 +++++
 src/readelf.c | 9 +++------
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/src/ChangeLog b/src/ChangeLog
index 1f53ca7b..9e30df31 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,8 @@
+2021-02-02 Timm Bäder <tbaeder@redhat.com>
+
+	* readelf.c (print_debug_line_section): Remove unnecessary
+	show_op_index variable, replace with (op_index > 0).
+
 2021-01-08  Timm Bäder  <tbaeder@redhat.com>
 
 	* readelf.c (print_cfa_program): Lift regname function to...
diff --git a/src/readelf.c b/src/readelf.c
index 9943c211..11692bb5 100644
--- a/src/readelf.c
+++ b/src/readelf.c
@@ -8752,14 +8752,11 @@ print_debug_line_section (Dwfl_Module *dwflmod, Ebl *ebl, GElf_Ehdr *ehdr,
       /* Apply the "operation advance" from a special opcode
 	 or DW_LNS_advance_pc (as per DWARF4 6.2.5.1).  */
       unsigned int op_addr_advance;
-      bool show_op_index;
       inline void advance_pc (unsigned int op_advance)
       {
 	op_addr_advance = minimum_instr_len * ((op_index + op_advance)
 					       / max_ops_per_instr);
 	address += op_addr_advance;
-	show_op_index = (op_index > 0 ||
-			 (op_index + op_advance) % max_ops_per_instr > 0);
 	op_index = (op_index + op_advance) % max_ops_per_instr;
       }
 
@@ -8803,7 +8800,7 @@ print_debug_line_section (Dwfl_Module *dwflmod, Ebl *ebl, GElf_Ehdr *ehdr,
 	      printf (_(" special opcode %u: address+%u = "),
 		      opcode, op_addr_advance);
 	      print_dwarf_addr (dwflmod, 0, address, address);
-	      if (show_op_index)
+	      if (op_index > 0)
 		printf (_(", op_index = %u, line%+d = %zu\n"),
 			op_index, line_increment, line);
 	      else
@@ -8921,7 +8918,7 @@ print_debug_line_section (Dwfl_Module *dwflmod, Ebl *ebl, GElf_Ehdr *ehdr,
 		    printf (_(" advance address by %u to "),
 			    op_addr_advance);
 		    print_dwarf_addr (dwflmod, 0, address, address);
-		    if (show_op_index)
+		    if (op_index > 0)
 		      printf (_(", op_index to %u"), op_index);
 		    printf ("\n");
 		  }
@@ -8982,7 +8979,7 @@ print_debug_line_section (Dwfl_Module *dwflmod, Ebl *ebl, GElf_Ehdr *ehdr,
 		    printf (_(" advance address by constant %u to "),
 			    op_addr_advance);
 		    print_dwarf_addr (dwflmod, 0, address, address);
-		    if (show_op_index)
+		    if (op_index > 0)
 		      printf (_(", op_index to %u"), op_index);
 		    printf ("\n");
 		  }
-- 
2.26.2

Re: [PATCH] readelf: Remove show_op_index variable

[Mark Wielaard]

Hi Timm,

On Tue, 2021-02-02 at 11:23 +0100, Timm Bäder via Elfutils-devel wrote:
> advance_pc() uses show_op_index to save whether the current op_index
> is
> > 0 OR the new op_index is > 0.
> 
> The new op index is calculated via
> 
> new_op_index = (op_index + op_advance) % max_ops_per_instr;
> 
> since all of the variables involved are unsigned,
> new_op_index >= op_index is always true.
> 
> So...
> 
> if op_index > 0, then new_op_index > 0
> if op_index == 0, then new_op_index >= 0
> 
> and if the new_op_index is > 0, then the old one was as well.
> 
> In any case, we only need to check the new_op_index, since show_op_index
> used to OR the two comparisons.
> 
> In other words:
> 
>  op_index > 0  |  new_op_index > 0  || show_op_index
>  ------------------------------------------------
>       true           true                true
>      false           true                true
>       true          false                true     xx
>      false          false                false
> 
> ... but since the third line (marked with xx) is not possible,
> the table becomes:
> 
>  op_index > 0  |  new_op_index > 0  || show_op_index
>  ------------------------------------------------
>       true           true                true
>      false           true                true
>      false          false                false
> 
> ... and show_op_index is equal to (new_op_index > 0).
> So, remove the show_op_index variable and simply replace it by comparing
> the new op_index > 0.

Very nice simplification. Thanks for the extensive commit message.

In most cases max_ops_per_instr is actually 1. In which case op_index
will always be zero.

Pushed to trunk.

Thanks,

Mark