https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93407
Bug ID: 93407
Summary: Dead partial memset not optimized away (clang does
that)
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: hubicka at gcc dot gnu.org
Target Milestone: ---
While analyzing GCC for PR93404 I noticed that gen_rtx_fmt_ee
34 rtx
35 gen_rtx_fmt_ee (code, mode, arg0, arg1)
36 RTX_CODE code;
37 enum machine_mode mode;
38 rtx arg0;
39 rtx arg1;
40 {
41 rtx rt;
42 rt = ggc_alloc_rtx (2);
43 memset (rt, 0, sizeof (struct rtx_def) - sizeof (rtunion));
44
45 PUT_CODE (rt, code);
46 PUT_MODE (rt, mode);
47 XEXP (rt, 0) = arg0;
48 XEXP (rt, 1) = arg1;
49
50 return rt;
51 }
gets quite odd codegen which gets inlined many times:
gen_rtx_fmt_ee():
/aux/hubicka/403.gcc/src/genrtl.c:41
4036ae: bf 18 00 00 00 mov $0x18,%edi
init_reload():
/aux/hubicka/403.gcc/src/reload1.c:500
4036b3: 41 0f 95 c6 setne %r14b
4036b7: 49 89 c5 mov %rax,%r13
gen_rtx_fmt_ee():
/aux/hubicka/403.gcc/src/genrtl.c:41
4036ba: e8 d1 64 0d 00 callq 4d9b90 <ggc_alloc>
init_reload():
/aux/hubicka/403.gcc/src/reload1.c:500
4036bf: 41 83 c6 04 add $0x4,%r14d
plus_constant_wide():
/aux/hubicka/403.gcc/src/genrtl.c:41
4036c3: be 04 00 00 00 mov $0x4,%esi
gen_rtx_fmt_ee():
/aux/hubicka/403.gcc/src/genrtl.c:42
4036c8: c7 40 03 00 00 00 00 movl $0x0,0x3(%rax)
/aux/hubicka/403.gcc/src/genrtl.c:41
4036cf: 48 89 c7 mov %rax,%rdi
/aux/hubicka/403.gcc/src/genrtl.c:42
4036d2: c6 40 07 00 movb $0x0,0x7(%rax)
/aux/hubicka/403.gcc/src/genrtl.c:44
4036d6: b8 4b 00 00 00 mov $0x4b,%eax
4036db: 66 89 07 mov %ax,(%rdi)
/aux/hubicka/403.gcc/src/genrtl.c:45
4036de: 44 88 77 02 mov %r14b,0x2(%rdi)
/aux/hubicka/403.gcc/src/genrtl.c:46
4036e2: 4c 89 6f 08 mov %r13,0x8(%rdi)
/aux/hubicka/403.gcc/src/genrtl.c:47
4036e6: 4c 89 67 10 mov %r12,0x10(%rdi)
For exmaple
4036c8: c7 40 03 00 00 00 00 movl $0x0,0x3(%rax)
seems to be leftover of the memset code. Clang does:
gen_rtx_fmt_ee():
/aux/hubicka/403.gcc/src/genrtl.c:43
67c72f: bf 18 00 00 00 mov $0x18,%edi
67c734: e8 a7 1a eb ff callq 52e1e0 <ggc_alloc>
67c739: 49 89 c6 mov %rax,%r14
/aux/hubicka/403.gcc/src/genrtl.c:44
67c73c: 48 c7 00 00 00 00 00 movq $0x0,(%rax)
/aux/hubicka/403.gcc/src/genrtl.c:46
67c743: 8d 85 4b 00 04 00 lea 0x4004b(%rbp),%eax
/aux/hubicka/403.gcc/src/genrtl.c:47
67c749: 41 89 06 mov %eax,(%r14)
/aux/hubicka/403.gcc/src/genrtl.c:48
67c74c: 49 89 5e 08 mov %rbx,0x8(%r14)
/aux/hubicka/403.gcc/src/genrtl.c:49
67c750: 4d 89 7e 10 mov %r15,0x10(%r14)
which simply fills in the rtx header and the two XEXP pointers.
