https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62173
--- Comment #27 from amker at gcc dot gnu.org ---
(In reply to Jiong Wang from comment #24)
> (In reply to amker from comment #23)
>
> partially agree.
>
> at least for the single use case given by Seb, I think tree ivopt should do
> it. (I verified clang do ivopt correctly for the case)
LLVM generates correct code, but I am not sure it's because of ivopt. The dump
after ivopt for llvm is like below:
; Function Attrs: nounwind
define void @bar(i32 %d) #0 {
entry:
%A = alloca [10 x i8], align 1
%cmp2 = icmp sgt i32 %d, 0
br i1 %cmp2, label %while.body.lr.ph, label %while.end
while.body.lr.ph: ; preds = %entry
%0 = sext i32 %d to i64
br label %while.body
while.body: ; preds = %while.body.lr.ph,
%while.body
%indvars.iv = phi i64 [ %0, %while.body.lr.ph ], [ %indvars.iv.next,
%while.body ]
%indvars.iv.next = add nsw i64 %indvars.iv, -1
%scevgep = getelementptr i8* %A4, i64 %indvars.iv
%1 = load i8* %scevgep, align 1, !tbaa !1
tail call void @foo(i8 %1) #2
%2 = add i64 %indvars.iv.next, 1
%cmp = icmp sgt i64 %2, 1
br i1 %cmp, label %while.body, label %while.end.loopexit
The induction variable chosen is the original biv (d) actually, just like GCC.
So even if we fix the idx_find_step issue, GCC's ivopt still can generate below
codes:
Loop-preheader
...
Loop-body:
iv = phi<d, -1>
tmp = (POINTER_TYPE)&A;
foo(MEM[base:tmp, index:iv]);
Without proper RTL optimization, very likely the issue in calculation of base
address of A still exists.
>
> for the rtl re-associate, it's a little bit painful from my experiment
> experiences. as it's not always good to reassociate virtual_frame + offset,
> we can only benefit if it's in loop, because the re-associate will increase
> register pressure, there will be situations that more callee-saved regs
> used, and finally we run into unncessary push/pop in pro/epilogue... and I
> haven't found a good place where we can safely re-use existed rtl info and
> do the rtl re-association as I am afraid rebuild those rtl info will cause
> compile time penalty.
