http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60738
Bug ID: 60738
Summary: A missing opportunity about
process_single_reg_class_operands
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: wmi at google dot com
Testcase 1.c:
int a, b, c, d, e, cond;
void foo() {
int r1, r2, r3;
r1 = b;
r2 = d;
if (__builtin_expect(cond > 3, 0)) {
e = e * 5;
c = a << r1;
}
c = c << r2;
__builtin_printf("r1 + r2 = %d\n", r1 + r2);
}
~/workarea/gcc-r208410/build/install/bin/gcc -O2 -S 1.c
foo:
.LFB0:
.cfi_startproc
cmpl $3, cond(%rip)
movl b(%rip), %esi
movl d(%rip), %eax
jg .L2
movl c(%rip), %edx
.L3:
movl %eax, %ecx // r2 gets assigned %eax. This is reload for
insn1.
addl %eax, %esi
movl $.LC0, %edi
sall %cl, %edx // insn1. Its constraint requires r2 in %ecx
xorl %eax, %eax
movl %edx, c(%rip)
jmp printf
.p2align 4,,10
.p2align 3
.L2:
movl e(%rip), %edx
movl %esi, %ecx // r1 gets assigned %esi. This is reload for
insn2.
leal (%rdx,%rdx,4), %edx
movl %edx, e(%rip)
movl a(%rip), %edx
sall %cl, %edx // insn2. Its constraint requires r1 in %ecx
jmp .L3
.cfi_endproc
Because the bb starting from L2 is relatively cold, it is better to generate
the code below:
foo:
.LFB0:
.cfi_startproc
cmpl $3, cond(%rip)
movl b(%rip), %esi
movl d(%rip), %ecx
jg .L2
movl c(%rip), %eax
.L3:
sall %cl, %eax // r2 gets assigned %ecx. no reload is
needed.
addl %ecx, %esi
movl $.LC0, %edi
movl %eax, c(%rip)
xorl %eax, %eax
jmp printf
.p2align 4,,10
.p2align 3
.L2:
movl e(%rip), %eax
movl %ecx, %edx // r2's live range is splitted here. This
is the start of the splitted live range.
movl %esi, %ecx // r1 gets assigned %esi, this is reload
for insn2.
leal (%rax,%rax,4), %eax
movl %eax, e(%rip)
movl a(%rip), %eax
sall %cl, %eax // insn2. constraint of insn2 requires r1
in %ecx
movl %edx, %ecx // r2's live range is splitted here. This
is the end of the splitted live range.
jmp .L3
.cfi_endproc
Now there is less code in the hotpath (The bb starting from .L3).
r1 and r2 used in sall insns need CX_REG class which is
single_reg_operand_class in IRA. Existing logic in
process_single_reg_class_operands in ira-lives.c doesn't allow %ecx to being
assigned to r1 or r2. May it need improvement here?
