var-tracking: Fix a wrong-debug issue caused by my r10-7665 var-tracking change [PR102441]

  Hi!

Since my r10-7665-g33c45e51b4914008064d9b77f2c1fc0eea1ad060 change, we get
wrong-debug on e.g. the following testcase at -O2 -g on x86_64-linux for the
x parameter:
void bar (int *r);
int
foo (int x)
{
  int r = 0;
  bar (&r);
  return r;
}
At the start of function, we have
        subq    $24, %rsp
        leaq    12(%rsp), %rdi
instructions.  The x parameter is passed in %rdi, but isn't used in the
function and so the leaq instruction overwrites %rdi without remembering
%rdi anywhere.  Before the r10-7665 change (which was trying to fix a large
(3% for 32-bit, 1% for 64-bit x86-64) debug info/loc growth introduced with
r10-7515), the leaq insn above resulted in a MO_VAL_SET micro-operation that
said that the value of sp + 12, a cselib_sp_derived_value_p, is stored into
the %rdi register.  The r10-7665 change added a change to add_stores that
added no micro-operation for the leaq store, with the rationale that the sp
based values can be and will be always computable some other more compact
and primarily more stable way (cfa based expression like DW_OP_fbreg, that
is the same in the whole function).  That is true.  But by throwing the
micro-operation on the floor, we miss another important part of the
MO_VAL_SET, in particular that the destination of the store, %rdi in this
case, now has a different value from what it had before, so the vt_*
dataflow code thinks that even after the leaq instruction %rdi still holds
the x argument value (and changes it to DW_OP_entry_value (%rdi) only in the
middle of the call to bar).  Previously and with the patches below,
the location for x changes already at the end of leaq instruction to
DW_OP_entry_value (%rdi).

My first attempt to fix this was instead of dropping the MO_VAL_SET add
a MO_CLOBBER operation:
so don't track that the value lives in the loc destination, but track
that the previous value doesn't live there anymore.  That failed bootstrap
miserably, the vt_* code isn't prepared to see MO_CLOBBER of a MEM that
isn't tracked (e.g. has MEM_EXPR on it that the var-tracking code wants
to track, i.e. track_p in add_stores).  On the other side, thinking about
it more, in the most common case where a cselib_sp_derived_value_p value
is stored into the sp register (and which is the reason why PR94495
testcase got larger), dropping the micro-operation on the floor is the
right thing, because we have that cselib_sp_derived_value_p tracking, any
reads from the sp hard register will be treated as
cselib_sp_derived_value_p.
Then I've tried 3 different patches, patch{1,2,3} attached below and all of
these passed bootstrap/regtest on x86_64-linux and i686-linux. 
Additionally, I've gathered statistics from cc1plus by always reverting the
var-tracking.c change after finished bootstrap/regtest and rebuilding the
stage3 var-tracking.o and cc1plus, such that it would be comparable.
dwlocstat and .debug_{info,loclists} section sizes detailed below.
patch3 uses MO_VAL_SET (i.e. essentially reversion of the r10-7665
change) when destination is not a REG_P and !track_p, otherwise if
destination is sp drops the micro-operation on the floor (i.e. no change),
otherwise adds a MO_CLOBBER.
patch1 is similar, except it checks for destination not equal to sp and
!track_p, i.e. for !track_p REG_P destinations other than sp it will use
MO_VAL_SET rather than MO_CLOBBER.
Finally, patch2, the shortest patch, uses MO_VAL_SET whenever destination
is not sp and otherwise drops the micro-operation on the floor.
All the 3 patches don't affect the PR94495 testcase, all the changes
there were caused by stores of sp based values into %rsp.

While the patch2 (and patch1 which results in exactly the same sizes)
causes the largest debug loclists/info growth from the 3, it is still quite
minor (0.651% on 64-bit and 0.114% on 32-bit) compared
to the 1% and 3% PR94495 was trying to solve, and I actually think it is the
best thing to do.  Because, if we have say
  int q[10];
  int *p = &q[0];
or similar and we load the &q[0] sp based value into some hard register,
by noting in the debug info that p lives in some hard reg for some part
of the function and a user is trying to change the p var in the debugger,
if we say it lives in some register or memory, there is some chance that
the changing of the value could work successfully (of course, nothing
is guaranteed, we don't have tracking of where each var lives at which
moment for changing purposes (i.e. what register, memory or else you need
to change in order to change behavior of the code)), while if we just say
that p's location is DW_OP_fbreg 16 DW_OP_stack_value, that is a read-only
value one can just print but not change.  Now, for stores of variable
values into the sp register, I don't think we have such an issue, you don't
want debugger to change your stack pointer when user asks to change value
of some variable whose value lives in the stack pointer, that would pretty
much always result in misbehavior of the program.
So, my preference from these 3 is patch2.

Ok for trunk (which one)?

64-bit cc1plus
==============
vanilla
cov%	samples	cumul
0..10	1064665/37%	1064665/37%
11..20	35972/1%	1100637/38%
21..30	47969/1%	1148606/40%
31..40	45787/1%	1194393/42%
41..50	57529/2%	1251922/44%
51..60	53974/1%	1305896/46%
61..70	112055/3%	1417951/50%
71..80	79420/2%	1497371/52%
81..90	126225/4%	1623596/57%
91..100	1206682/42%	2830278/100%
  [34] .debug_info       PROGBITS        0000000000000000 2f1c74c a44949f 00      0   0  1
  [38] .debug_loclists   PROGBITS        0000000000000000 ff5d046 506e947 00      0   0  1
patch1 (same as patch2)
cov%	samples	cumul
0..10	1064685/37%	1064685/37%
11..20	36011/1%	1100696/38%
21..30	47975/1%	1148671/40%
31..40	45799/1%	1194470/42%
41..50	57566/2%	1252036/44%
51..60	54011/1%	1306047/46%
61..70	112068/3%	1418115/50%
71..80	79421/2%	1497536/52%
81..90	126171/4%	1623707/57%
91..100	1206571/42%	2830278/100%
  [34] .debug_info       PROGBITS        0000000000000000 2f1c74c a448f27 00      0   0  1
  [38] .debug_loclists   PROGBITS        0000000000000000 ff608bc 52070dd 00      0   0  1
patch3
cov%	samples	cumul
0..10	1064698/37%	1064698/37%
11..20	36018/1%	1100716/38%
21..30	47977/1%	1148693/40%
31..40	45804/1%	1194497/42%
41..50	57562/2%	1252059/44%
51..60	54018/1%	1306077/46%
61..70	112071/3%	1418148/50%
71..80	79424/2%	1497572/52%
81..90	126172/4%	1623744/57%
91..100	1206534/42%	2830278/100%
  [34] .debug_info       PROGBITS        0000000000000000 2f1c74c a449548 00      0   0  1
  [38] .debug_loclists   PROGBITS        0000000000000000 ff5df39 507acd8 00      0   0  1
So, size of .debug_info+.debug_loclists grows for vanilla -> patch1 (or patch2) by
0.651% and for vanilla -> patch3 by 0.020%.

32-bit cc1plus
==============
vanilla
cov%	samples	cumul
0..10	1061892/37%	1061892/37%
11..20	34002/1%	1095894/39%
21..30	43513/1%	1139407/40%
31..40	41667/1%	1181074/42%
41..50	59144/2%	1240218/44%
51..60	47009/1%	1287227/45%
61..70	105069/3%	1392296/49%
71..80	72990/2%	1465286/52%
81..90	125988/4%	1591274/56%
91..100	1208726/43%	2800000/100%
  [33] .debug_info       PROGBITS        00000000 351ab10 8b1c83d 00      0   0  1
  [37] .debug_loclists   PROGBITS        00000000 ebc816e 3fe44fd 00      0   0  1
patch1 (same as patch2)
cov%	samples	cumul
0..10	1061999/37%	1061999/37%
11..20	34065/1%	1096064/39%
21..30	43557/1%	1139621/40%
31..40	41690/1%	1181311/42%
41..50	59191/2%	1240502/44%
51..60	47143/1%	1287645/45%
61..70	105045/3%	1392690/49%
71..80	73021/2%	1465711/52%
81..90	125885/4%	1591596/56%
91..100	1208404/43%	2800000/100%
  [33] .debug_info       PROGBITS        00000000 351ab10 8b1c597 00      0   0  1
  [37] .debug_loclists   PROGBITS        00000000 ebca915 401ffad 00      0   0  1
patch3
cov%	samples	cumul
0..10	1062006/37%	1062006/37%
11..20	34073/1%	1096079/39%
21..30	43559/1%	1139638/40%
31..40	41693/1%	1181331/42%
41..50	59189/2%	1240520/44%
51..60	47142/1%	1287662/45%
61..70	105054/3%	1392716/49%
71..80	73027/2%	1465743/52%
81..90	125874/4%	1591617/56%
91..100	1208383/43%	2800000/100%
  [33] .debug_info       PROGBITS        00000000 351ab10 8b1c690 00      0   0  1
  [37] .debug_loclists   PROGBITS        00000000 ebca40a 4020a6e 00      0   0  1
So, size of .debug_info+.debug_loclists grows for vanilla -> patch1 (or patch2) by
0.114% and for vanilla -> patch3 by 0.116%.

	Jakub
2021-10-04  Jakub Jelinek  <jakub@redhat.com>

	PR debug/102441
	* var-tracking.c (add_stores): For cselib_sp_derived_value_p values
	use MO_VAL_SET if !track_p and loc is not sp, otherwise emit a
	MO_CLOBBER microoperation unless loc is sp.

--- gcc/var-tracking.c.jj	2021-09-27 23:34:52.290324697 +0200
+++ gcc/var-tracking.c	2021-10-01 18:51:07.539130485 +0200
@@ -6129,11 +6129,16 @@ add_stores (rtx loc, const_rtx expr, voi
      on which register holds that VALUE in some instruction.  */
   if (!frame_pointer_needed
       && cfa_base_rtx
-      && cselib_sp_derived_value_p (v))
+      && cselib_sp_derived_value_p (v)
+      && (loc == stack_pointer_rtx || track_p))
     {
       if (preserve)
 	preserve_value (v);
-      return;
+      if (loc == stack_pointer_rtx)
+	return;
+      mo.type = MO_CLOBBER;
+      mo.u.loc = loc;
+      goto log_and_return;
     }
 
   nloc = replace_expr_with_values (oloc);
2021-10-04  Jakub Jelinek  <jakub@redhat.com>

	PR debug/102441
	* var-tracking.c (add_stores): For cselib_sp_derived_value_p values
	use MO_VAL_SET if loc is not sp.

--- gcc/var-tracking.c.jj	2021-09-27 23:34:52.290324697 +0200
+++ gcc/var-tracking.c	2021-10-02 21:46:54.420433439 +0200
@@ -6129,7 +6129,8 @@ add_stores (rtx loc, const_rtx expr, voi
      on which register holds that VALUE in some instruction.  */
   if (!frame_pointer_needed
       && cfa_base_rtx
-      && cselib_sp_derived_value_p (v))
+      && cselib_sp_derived_value_p (v)
+      && loc == stack_pointer_rtx)
     {
       if (preserve)
 	preserve_value (v);
2021-10-04  Jakub Jelinek  <jakub@redhat.com>

	PR debug/102441
	* var-tracking.c (add_stores): For cselib_sp_derived_value_p values
	use MO_VAL_SET if !track_p and loc is not a REG, otherwise emit a
	MO_CLOBBER microoperation unless loc is sp.

--- gcc/var-tracking.c.jj	2021-09-27 23:34:52.290324697 +0200
+++ gcc/var-tracking.c	2021-10-03 21:29:21.683588426 +0200
@@ -6129,11 +6129,16 @@ add_stores (rtx loc, const_rtx expr, voi
      on which register holds that VALUE in some instruction.  */
   if (!frame_pointer_needed
       && cfa_base_rtx
-      && cselib_sp_derived_value_p (v))
+      && cselib_sp_derived_value_p (v)
+      && (REG_P (loc) || track_p))
     {
       if (preserve)
 	preserve_value (v);
-      return;
+      if (loc == stack_pointer_rtx)
+	return;
+      mo.type = MO_CLOBBER;
+      mo.u.loc = loc;
+      goto log_and_return;
     }
 
   nloc = replace_expr_with_values (oloc);
  

Hello, Jakub,

On Oct  4, 2021, Jakub Jelinek <jakub@redhat.com> wrote:

> Finally, patch2, the shortest patch, uses MO_VAL_SET whenever destination
> is not sp and otherwise drops the micro-operation on the floor.

That sounds quite reasonable to me, and it is indeed my favorite of the
3 proposed patches, because the mapping of locations to values is kept
most accurate.

Thanks!
  

On October 9, 2021 5:26:17 AM GMT+02:00, Alexandre Oliva <oliva@adacore.com> wrote:
>Hello, Jakub,
>
>On Oct  4, 2021, Jakub Jelinek <jakub@redhat.com> wrote:
>
>> Finally, patch2, the shortest patch, uses MO_VAL_SET whenever destination
>> is not sp and otherwise drops the micro-operation on the floor.
>
>That sounds quite reasonable to me, and it is indeed my favorite of the
>3 proposed patches, because the mapping of locations to values is kept
>most accurate.

What I thought as well. Thus, OK. 

Richard. 


>Thanks!
>