Bug 80054

Summary: [7 Regression] ICE in verify_ssa with -O3 -march=broadwell/skylake-avx512
Product: gcc Reporter: Vsevolod Livinskii <vsevolod.livinskiy>
Component: tree-optimizationAssignee: Bill Schmidt <bill.schmidt>
Status: RESOLVED FIXED    
Severity: normal CC: bill.schmidt
Priority: P1 Keywords: ice-on-valid-code
Version: 7.0.1   
Target Milestone: 7.0   
Host: Target:
Build: Known to work: 6.3.1
Known to fail: Last reconfirmed: 2017-03-16 00:00:00
Bug Depends on:    
Bug Blocks: 103035    
Attachments: Reproducer.

Description Vsevolod Livinskii 2017-03-15 17:13:04 UTC 
Created attachment 40978 [details]
Reproducer.

ICE with -O3 -march=broadwell (and skylake-avx512). Everything works fine with other optimization levels.

Error:
>$ g++ -O3 -march=broadwell -c repr.cpp
repr.cpp: In function ‘void foo()’:
repr.cpp:17:6: error: definition in block 7 does not dominate use in block 6
 void foo() {
      ^~~
for SSA_NAME: _133 in statement:
slsr_142 = PHI <_133(6), _133(16)>
PHI argument
_133
for PHI node
slsr_142 = PHI <_133(6), _133(16)>
repr.cpp:17:6: internal compiler error: verify_ssa failed
0xf84ad3 verify_ssa(bool, bool)
	/home/vsevolod/workspace/gcc-dev/trunk/gcc/tree-ssa.c:1184
0xc98ea7 execute_function_todo
	/home/vsevolod/workspace/gcc-dev/trunk/gcc/passes.c:1973
0xc99e0b execute_todo
	/home/vsevolod/workspace/gcc-dev/trunk/gcc/passes.c:2016

Reproducer:
extern short var_2;
extern short var_4;
extern const bool var_32;
extern short var_36;
extern const bool var_37;
extern bool var_46;
extern unsigned int var_47;
extern short var_49;
extern unsigned int var_56;
extern unsigned int var_62;
extern unsigned int var_65;
extern bool var_831;
extern unsigned int var_843;
extern short var_846;
extern short var_889;

void foo() {
    if (var_36 * var_37)
        var_831 = var_56 = 0;
    else
        var_65 = 0;
    
    if (var_46)
        var_843 = 0;
    
    var_846 = 0;
    
    if ((var_4 == 0) >> (var_32 | -(var_37 < var_46 || var_36)) + 8)
        var_49 = 2032651381 * bool(var_2 * var_37);
    else {
        var_62 = 0;
        var_47 = (var_46 || var_36) * (var_2 * var_37);
    }
    
    var_889 = bool(var_2 * var_37);
}

GCC version:
gcc version 7.0.1 (today's trunk)
Comment 1 Richard Biener 2017-03-16 14:57:19 UTC 
Confirmed.  SLSR is at fault.
Comment 2 Bill Schmidt 2017-03-16 14:59:29 UTC 
OK, I'll have a look.
Comment 3 Bill Schmidt 2017-03-16 15:58:31 UTC 
Reproducible on ppc64le without any -march.
Comment 4 Bill Schmidt 2017-03-16 20:23:32 UTC 
PRE creates a situation where a conditional SLSR candidate depends on a PHI that occurs prior to the basis for the candidate.  SLSR doesn't notice this and eventually creates a phi basis that is not dominated by the true basis, leading to a use of an undefined variable.

The fix will be to test whether the basis dominates all PHIs and their PHI argument definitions, and avoid the optimization in such cases.  Testing now.
Comment 5 Bill Schmidt 2017-03-20 20:04:58 UTC 
Author: wschmidt
Date: Mon Mar 20 20:04:25 2017
New Revision: 246290

URL: https://gcc.gnu.org/viewcvs?rev=246290&root=gcc&view=rev
Log:
[gcc]

2017-03-20  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>

	PR tree-optimization/80054
	* gimple-ssa-strength-reduction.c (all_phi_incrs_profitable): Fail
	the optimization if a PHI or any of its arguments is not dominated
	by the candidate's basis.  Use gphi* rather than gimple* as
	appropriate.
	(replace_profitable_candidates): Clean up a gimple* variable that
	should be a gphi* variable.

[gcc/testsuite]

2017-03-20  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>

	PR tree-optimization/80054
	* g++.dg/torture/pr80054.C: New file.


Added:
    trunk/gcc/testsuite/g++.dg/torture/pr80054.C
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/gimple-ssa-strength-reduction.c
    trunk/gcc/testsuite/ChangeLog
Comment 6 Bill Schmidt 2017-03-20 20:05:53 UTC 
Fixed.