Hi!
As I wrote in the PR, vt_stack_adjustments can often compute wrong offsets,
because it never considers pops with autoinc addressing, which can lead
either to wrong debug info, or turning off -fvar-tracking altogether for
a function on which that issue resulted in stack depth inconsistencies on
the edges.
Here are some stats from --enable-checking=yes,rtl cc1plus bootstrapped
with/without the patch (without the patch got then rebuilt stage3 with the
patch, i.e. just var-tracking.o and cc1plus-checksum.o got recompiled, so
I'm comparing identical code, different debug info):
x86_64-linux cc1plus built without the patch:
cov% samples cumul
0.0 506230/38% 506230/38%
0..10 10327/0% 516557/39%
11..20 12390/0% 528947/39%
21..30 31265/2% 560212/42%
31..40 18775/1% 578987/43%
41..50 20631/1% 599618/45%
51..60 24921/1% 624539/47%
61..70 40959/3% 665498/50%
71..80 23771/1% 689269/52%
81..90 41771/3% 731040/55%
91..99 81667/6% 812707/61%
100 510564/38% 1323271/100%
x86_64-linux cc1plus built with the patch:
cov% samples cumul
0.0 382214/28% 382214/28%
0..10 13100/0% 395314/29%
11..20 14568/1% 409882/30%
21..30 33708/2% 443590/33%
31..40 21927/1% 465517/35%
41..50 23924/1% 489441/36%
51..60 28736/2% 518177/39%
61..70 45847/3% 564024/42%
71..80 29284/2% 593308/44%
81..90 52085/3% 645393/48%
91..99 99971/7% 745364/56%
100 577907/43% 1323271/100%
i686-linux cc1plus built without the patch:
cov% samples cumul
0.0 631348/48% 631348/48%
0..10 7764/0% 639112/48%
11..20 9690/0% 648802/49%
21..30 25036/1% 673838/51%
31..40 16113/1% 689951/52%
41..50 19753/1% 709704/54%
51..60 14563/1% 724267/55%
61..70 34093/2% 758360/58%
71..80 17450/1% 775810/59%
81..90 31339/2% 807149/61%
91..99 60368/4% 867517/66%
100 437548/33% 1305065/100%
i686-linux cc1plus built with the patch:
cov% samples cumul
0.0 377352/28% 377352/28%
0..10 16077/1% 393429/30%
11..20 15390/1% 408819/31%
21..30 31790/2% 440609/33%
31..40 23889/1% 464498/35%
41..50 29267/2% 493765/37%
51..60 22902/1% 516667/39%
61..70 45629/3% 562296/43%
71..80 29511/2% 591807/45%
81..90 50536/3% 642343/49%
91..99 93584/7% 735927/56%
100 569138/43% 1305065/100%
.debug_info/.debug_loc sizes in bytes:
x86_64-linux cc1plus without patch .debug_info 75411710, .debug_loc 75421077
x86_64-linux cc1plus with patch .debug_info 78498790, .debug_loc 90530117
i686-linux cc1plus without patch .debug_info 59921183, .debug_loc 37823166
i686-linux cc1plus with patch .debug_info 63009554, .debug_loc 59535100
I've also performed instrumented bootstraps/regtests (x86_64-linux and
i686-linux), where I've logged in how many functions the result of
vt_stack_adjustments differed between the bad old way and new way.
In both the bootstraps/regtests, it affected 16892 32-bit and 6646 64-bit
functions, in all cases it was old way giving up and new way succeeding.
Not adding a testcase, as the one in the PR failed to produce proper debug
info only in 4.8 (then got latent), there already are some guality improvements with the
patch:
-FAIL: gcc.dg/guality/pr54693-2.c -O3 -fomit-frame-pointer -funroll-all-loops -finline-functions line 21 i == v + 1
-FAIL: gcc.dg/guality/pr54693-2.c -O3 -fomit-frame-pointer -funroll-loops line 21 i == v + 1
on x86_64 and:
-FAIL: gcc.dg/guality/pr54519-1.c -O2 line 20 y == 25
-FAIL: gcc.dg/guality/pr54519-1.c -O2 line 20 z == 6
-FAIL: gcc.dg/guality/pr54519-1.c -O2 line 23 y == 117
-FAIL: gcc.dg/guality/pr54519-1.c -O2 line 23 z == 8
-FAIL: gcc.dg/guality/pr54519-1.c -O3 -fomit-frame-pointer line 20 x == 36
-FAIL: gcc.dg/guality/pr54519-1.c -O3 -fomit-frame-pointer line 20 y == 25
-FAIL: gcc.dg/guality/pr54519-1.c -O3 -fomit-frame-pointer line 20 z == 6
-FAIL: gcc.dg/guality/pr54519-1.c -O3 -g line 20 x == 36
-FAIL: gcc.dg/guality/pr54519-1.c -O3 -g line 20 y == 25
-FAIL: gcc.dg/guality/pr54519-1.c -O3 -g line 20 z == 6
-FAIL: gcc.dg/guality/pr54519-3.c -O2 line 20 x == 36
-FAIL: gcc.dg/guality/pr54519-3.c -O2 line 20 y == 25
-FAIL: gcc.dg/guality/pr54519-3.c -O2 line 20 z == 6
-FAIL: gcc.dg/guality/pr54519-3.c -O2 line 23 x == 98
-FAIL: gcc.dg/guality/pr54519-3.c -O2 line 23 y == 117
-FAIL: gcc.dg/guality/pr54519-3.c -O2 line 23 z == 8
-FAIL: gcc.dg/guality/pr54519-3.c -O2 -flto line 20 x == 36
-FAIL: gcc.dg/guality/pr54519-3.c -O2 -flto line 23 x == 98
-FAIL: gcc.dg/guality/pr54519-3.c -O2 -flto -flto-partition=none line 20 x == 36
-FAIL: gcc.dg/guality/pr54519-3.c -O2 -flto -flto-partition=none line 23 x == 98
-FAIL: gcc.dg/guality/pr54519-3.c -O3 -fomit-frame-pointer line 20 x == 36
-FAIL: gcc.dg/guality/pr54519-3.c -O3 -fomit-frame-pointer line 20 y == 25
-FAIL: gcc.dg/guality/pr54519-3.c -O3 -fomit-frame-pointer line 20 z == 6
-FAIL: gcc.dg/guality/pr54519-3.c -O3 -fomit-frame-pointer line 23 x == 98
-FAIL: gcc.dg/guality/pr54519-3.c -O3 -fomit-frame-pointer line 23 y == 117
-FAIL: gcc.dg/guality/pr54519-3.c -O3 -fomit-frame-pointer line 23 z == 8
-FAIL: gcc.dg/guality/pr54519-3.c -O3 -g line 20 x == 36
-FAIL: gcc.dg/guality/pr54519-3.c -O3 -g line 20 y == 25
-FAIL: gcc.dg/guality/pr54519-3.c -O3 -g line 20 z == 6
-FAIL: gcc.dg/guality/pr54519-3.c -O3 -g line 23 x == 98
-FAIL: gcc.dg/guality/pr54519-3.c -O3 -g line 23 y == 117
-FAIL: gcc.dg/guality/pr54519-3.c -O3 -g line 23 z == 8
-FAIL: gcc.dg/guality/pr54693-2.c -O3 -fomit-frame-pointer -funroll-all-loops -finline-functions line 21 i == v + 1
-FAIL: gcc.dg/guality/pr54693-2.c -O3 -fomit-frame-pointer -funroll-loops line 21 i == v + 1
on i686.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
Not sure about branches, to some extent this must be a serious debug info
quality regression, since the problem got significantly worse with omitting
frame pointer by default on i686, and/or shrink wrapping and simple_return
changes where there are more pops in the middle of functions. So with a short
patch we could significantly improve debug info. The risk is compile time
regressions, if we gave up on var-tracking, we threw away all the hard tracked
debug stmts/debug insns, but don't need to costly propagate the values through
the dataflow.
2014-10-23 Jakub Jelinek <jakub@redhat.com>
PR debug/63623
* var-tracking.c (stack_adjust_offset_pre_post_cb): New function.
(stack_adjust_offset_pre_post): Use it through for_each_inc_dec,
instead of only handling autoinc in dest if it is a MEM.
(vt_stack_adjustments): Fix up formatting.