https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95798
Bug ID: 95798
Summary: Initialization code --- suboptimal
Product: gcc
Version: 9.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: zero at smallinteger dot com
Target Milestone: ---
Created attachment 48764
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48764&action=edit
sample code
This is similar to (but not the same as) bug 87223 for structs. Further, this
bug expands on this issue for gcc 10.x. Originally, this was noted in gcc
(Ubuntu 9.3.0-10ubuntu2) 9.3.0, compiling with -O3.
First, note the initialization code that trivially sets values to zero in an
array,
mov eax, edi
sub rsp, 8080
xor edx, edx
and eax, 127
mov QWORD PTR [rsp-120+rax*8], 0
mov QWORD PTR [rsp-112+rax*8], 0
mov QWORD PTR [rsp-104+rax*8], 0
mov QWORD PTR [rsp-96+rax*8], 0
mov QWORD PTR [rsp-88+rax*8], 0
mov QWORD PTR [rsp-80+rax*8], 0
mov QWORD PTR [rsp-72+rax*8], 0
mov QWORD PTR [rsp-64+rax*8], 0
xor eax, eax
would be better by first setting a register to zero, then writing the value of
the register. Further, note that there is already a zero register available
(edx), but it is not used. This is similar to 87223 for structs, and here the
issue manifests for arrays.
Second, using gcc 10 versions and -O3 at godbolt.org results in this code:
mov eax, edi
mov edx, edi
sub rsp, 8072
and eax, 127
and edx, 127
mov QWORD PTR [rsp-120+rdx*8], 0
lea edx, [rax+1]
movsx rdx, edx
mov QWORD PTR [rsp-120+rdx*8], 0
lea edx, [rax+2]
movsx rdx, edx
mov QWORD PTR [rsp-120+rdx*8], 0
lea edx, [rax+3]
movsx rdx, edx
mov QWORD PTR [rsp-120+rdx*8], 0
lea edx, [rax+4]
movsx rdx, edx
mov QWORD PTR [rsp-120+rdx*8], 0
lea edx, [rax+5]
movsx rdx, edx
mov QWORD PTR [rsp-120+rdx*8], 0
lea edx, [rax+6]
add eax, 7
movsx rdx, edx
cdqe
mov QWORD PTR [rsp-120+rdx*8], 0
xor edx, edx
mov QWORD PTR [rsp-120+rax*8], 0
xor eax, eax
This is much, much more verbose than in gcc 9.3, for no apparent gain.
