[Bug middle-end/30907] New: [dataflow] Bad interaction with addressing mode selection and regalloc

[rguenth at gcc dot gnu dot org]

int bar (void);
void foo (int *);
static int s[10];

void foobar (int i1, int i2, int i3, int i4, int i5, int i6)
{
  int a[100];
  int i, i7;

  i7 = bar ();
  bar ();

  for (i = 0; i < 100; i++)
    a[i] = s[i1] + s[i2] + s[i3] + s[i4] + s[i5] + s[i6] + s[i7];

  foo (&a[0]);

  return;
}

If you compare mainline to dataflow branch at -O2 you can see

--- t.i.trunk   2007-02-21 11:31:09.663252586 +0100
+++ t.i.df      2007-02-21 11:31:10.548064364 +0100
@@ -37,7 +37,6 @@
        movl    s(,%rbx,4), %edx
        addl    s(,%rcx,4), %edx
        movslq  %r12d,%r12
-       leaq    16(%rsp), %rdi
        addl    s(,%r13,4), %edx
        addl    s(,%r14,4), %edx
        addl    s(,%r15,4), %edx
@@ -46,10 +45,11 @@
        addl    s(,%r12,4), %edx
        .p2align 4,,7
 .L2:
-       movl    %edx, (%rdi,%rax,4)
+       movl    %edx, 16(%rsp,%rax,4)
        addq    $1, %rax
        cmpq    $100, %rax
        jne     .L2
+       leaq    16(%rsp), %rdi
        call    foo
        addq    $424, %rsp
        popq    %rbx

that is, we are choosing a more expensive addressing mode in the loop not
noticing that 16(%rsp) can be (G)CSEd.  This makes the above loop run
33% slower on x86_64.


-- 
           Summary: [dataflow] Bad interaction with addressing mode
                    selection and regalloc
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rguenth at gcc dot gnu dot org
GCC target triplet: x86_64-*-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30907