https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86844
Bug ID: 86844
Summary: wrong code generation cause by store merging pass
Product: gcc
Version: 8.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: krebbel at gcc dot gnu.org
Target Milestone: ---
Created attachment 44502
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44502&action=edit
Reduced testcase
Compiling the attached testcase with -O2 results in the following code:
movzbl 4(%rdi), %eax
movl $33024, 8(%rdi)
movb %al, 10(%rdi)
33024 -> 0 0 129 0
The store of 222 gets optimized away.
Without store merging:
movzbl 4(%rdi), %eax
movl $0, 8(%rdi)
movb $-34, 11(%rdi)
movb $-127, 9(%rdi)
movb %al, 10(%rdi)
The original order of stores:
a->b.wd0.u4i = 0;
a->b.wd0.s2.w = 222;
a->b.wd0.s2.y = 129;
a->b.wd0.s2.z = a->f.wd1.s2.z;
coalesce_immediate_stores first reorders the stores according to its bit
positions:
a->b.wd0.u4i = 0;
a->b.wd0.s2.y = 129;
a->b.wd0.s2.z = a->f.wd1.s2.z;
a->b.wd0.s2.w = 222;
It then merges the first and the second and has to end the group seeing the
third. So the last ends up in its own group. Emitting the stores in the
original order makes the 222 store dead. The first two should not be merged.
coalesce_immediate_stores already tries to detect cases where stores later in
the chain might get invalidated by merging early stores but it also assumes
that if the later store also stores a constant it will be possible to merge it
as well. However, in this case the non-constant store in between prevents
this.
Store merging pass output:
;; Function f (f, funcdef_no=0, decl_uid=1922, cgraph_uid=1, symbol_order=0)
Processing basic block <2>:
Starting new chain with statement:
a_3(D)->b.wd0.u4i = 0;
The base object is:
a_3(D)
Recording immediate store from stmt:
a_3(D)->b.wd0.s2.w = 222;
Recording immediate store from stmt:
a_3(D)->b.wd0.s2.y = 129;
Recording immediate store from stmt:
a_3(D)->b.wd0.s2.z = _1;
stmt causes chain termination:
return;
Attempting to coalesce 4 stores in chain
New store group
Store 0:
bitsize:32 bitpos:64 val:0
Store 1:
bitsize:8 bitpos:72 val:129
After writing 0 of size 32 at position 0
the merged value contains 00 00 00 00
the merged mask contains 00 00 00 00
After writing 129 of size 8 at position 8
the merged value contains 00 81 00 00
the merged mask contains 00 00 00 00
New store group
Store 2:
bitsize:8 bitpos:80 val:_1
New store group
Store 3:
bitsize:8 bitpos:88 val:222
Coalescing successful!
Merged into 1 stores
New sequence of 1 stores to replace old one of 2 stores
# .MEM_6 = VDEF <.MEM_5>
MEM[(union *)a_3(D) + 8B] = 33024;
Merging successful!
f (struct bar * a)
{
unsigned char _1;
<bb 2> [local count: 1073741825]:
a_3(D)->b.wd0.s2.w = 222;
MEM[(union *)a_3(D) + 8B] = 33024;
_1 = a_3(D)->D.1919.f.wd1.s2.z;
a_3(D)->b.wd0.s2.z = _1;
return;
}
