[llvm-dev] SCEV and <nsw>
Alexandre Isoard via llvm-dev
llvm-dev at lists.llvm.org
Fri Oct 26 11:12:42 PDT 2018
I finally managed to reproduce the issue:
define void @bad(i32* %A, i32 %x) {
entry:
%do = icmp slt i32 0, %x
br i1 %do, label %loop.prehead, label %exit
loop.prehead: ; preds = %entry
br label %loop.body
loop.body: ; preds = %loop.body,
%loop.prehead
%i = phi i32 [ 0, %loop.prehead ], [ %i.next, %loop.body ]
%i.next = add nsw i32 %i, 1
%i.96 = add nsw i32 %i, 96
* %store.addr = getelementptr i32, i32* %A, i32 %i.next*
store i32 %i, i32* %store.addr, align 4
%cmp = icmp slt i32 %i.next, %x
br i1 %cmp, label %loop.body, label %loop.exit
loop.exit: ; preds = %loop.body
br label %exit
exit: ; preds = %loop.exit,
%entry
ret void
}
; Printing analysis 'Scalar Evolution Analysis' for function 'bad':
; Classifying expressions for: @bad
; %i = phi i32 [ 0, %loop.prehead ], [ %i.next, %loop.body ]
; --> {0,+,1}<nuw><nsw><%loop.body> U: [0,2147483647) S:
[0,2147483647) Exits:
(-1 + %x) LoopDispositions: { %loop.body: Computable }
; %i.next = add nsw i32 %i, 1
; --> {1,+,1}<nuw><nsw><%loop.body> U: [1,-2147483648) S:
[1,-2147483648) Exits:
%x LoopDispositions: { %loop.body: Computable }
; %i.96 = add nsw i32 %i, 96
; --> {96,+,1}<nuw><%loop.body> U: [96,-2147483553) S:
[96,-2147483553) Exits:
(95 + %x) LoopDispositions: { %loop.body: Computable }
; %store.addr = getelementptr i32, i32* %A, i32 %i.next
; --> {(8 + %A),+,8}<%loop.body> U: full-set S: full-set Exits: (8 + (8
* (zext i32 (-1 + %x) to i64)) + %A) LoopDispositions: { %loop.body:
Computable }
; Determining loop execution counts for: @bad
; Loop %loop.body: backedge-taken count is (-1 + %x)
; Loop %loop.body: max backedge-taken count is 2147483646
; Loop %loop.body: Predicated backedge-taken count is (-1 + %x)
; Predicates:
;
; Loop %loop.body: Trip multiple is 1
define void @good(i32* %A, i32 %x) {
entry:
%do = icmp slt i32 0, %x
br i1 %do, label %loop.prehead, label %exit
loop.prehead: ; preds = %entry
br label %loop.body
loop.body: ; preds = %loop.body,
%loop.prehead
%i = phi i32 [ 0, %loop.prehead ], [ %i.next, %loop.body ]
%i.next = add nsw i32 %i, 1
%i.96 = add nsw i32 %i, 96
* %store.addr = getelementptr i32, i32* %A, i32 %i.96*
store i32 %i, i32* %store.addr, align 4
%cmp = icmp slt i32 %i.next, %x
br i1 %cmp, label %loop.body, label %loop.exit
loop.exit: ; preds = %loop.body
br label %exit
exit: ; preds = %loop.exit,
%entry
ret void
}
; Printing analysis 'Scalar Evolution Analysis' for function 'good':
; Classifying expressions for: @good
; %i = phi i32 [ 0, %loop.prehead ], [ %i.next, %loop.body ]
; --> {0,+,1}<nuw><nsw><%loop.body> U: [0,2147483647) S:
[0,2147483647) Exits:
(-1 + %x) LoopDispositions: { %loop.body: Computable }
; %i.next = add nsw i32 %i, 1
; --> {1,+,1}<nuw><nsw><%loop.body> U: [1,-2147483648) S:
[1,-2147483648) Exits:
%x LoopDispositions: { %loop.body: Computable }
; %i.96 = add nsw i32 %i, 96
; --> {96,+,1}<nuw><nsw><%loop.body> U: [96,-2147483648) S:
[96,-2147483648) Exits: (95 + %x) LoopDispositions: { %loop.body:
Computable }
; %store.addr = getelementptr i32, i32* %A, i32 %i.96
; --> {(768 + %A),+,8}<%loop.body> U: full-set S: full-set Exits: (768 +
(8 * (zext i32 (-1 + %x) to i64)) + %A) LoopDispositions: { %loop.body:
Computable }
; Determining loop execution counts for: @good
; Loop %loop.body: backedge-taken count is (-1 + %x)
; Loop %loop.body: max backedge-taken count is 2147483646
; Loop %loop.body: Predicated backedge-taken count is (-1 + %x)
; Predicates:
;
; Loop %loop.body: Trip multiple is 1
Notice the only difference is on the gep (in bold). Can somebody shed some
light?
I attached the test file.
On Fri, Oct 26, 2018 at 10:41 AM Alexandre Isoard <
alexandre.isoard at gmail.com> wrote:
> Hello,
>
> I'm running into an issue where SCEV misses explicit nsw flags on the
> following expressions:
>
> %mm.07 = phi i32 [ 0, %for.body.lr.ph ], [ %add, %for.inc30 ]
> --> {0,+,1}<nuw><nsw><%for.body> U: [0,2147483647) S: [0,2147483647) Exits:
> (-1 + %m) LoopDispositions: { %for.body: Computable, %for.body5:
> Invariant, %for.inc: Invariant }
> %add = add nsw i32 %mm.07, 1
> --> {1,+,1}<nuw><nsw><%for.body> U: [1,-2147483648) S: [1,-2147483648) Exits:
> %m LoopDispositions: { %for.body: Computable, %for.body5: Invariant,
> %for.inc: Invariant }
> %add2 = add nsw i32 %mm.07, 96
> --> {96,+,1}<nuw><%for.body> U: [96,-2147483553) S: [96,-2147483553) Exits:
> (95 + %m) LoopDispositions: { %for.body: Computable, %for.body5:
> Invariant, %for.inc: Invariant }
>
> My problem is with the later one, where the <nsw> is missing (which cause
> me problems down the line with gep computation on 64 bit address space).
>
> Any clue as to what could be the source of that disappearance? I tried to
> reproduce the issue on simple cases but to no avail. I get the following
> expected result:
>
> %i = phi i32 [ 0, %loop.prehead ], [ %i.next, %loop.body ]
> --> {0,+,1}<nuw><nsw><%loop.body> U: [0,2147483647) S: [0,2147483647) Exits:
> (-1 + %x) LoopDispositions: { %loop.body: Computable }
> %i.next = add nsw i32 %i, 1
> --> {1,+,1}<nuw><nsw><%loop.body> U: [1,-2147483648) S: [1,-2147483648) Exits:
> %x LoopDispositions: { %loop.body: Computable }
> %i.96 = add nsw i32 %i, 96
> --> {96,+,1}<nuw><nsw><%loop.body> U: [96,-2147483648) S:
> [96,-2147483648) Exits: (95 + %x) LoopDispositions: { %loop.body:
> Computable }
>
> Help?
>
> --
> *Alexandre Isoard*
>
--
*Alexandre Isoard*
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181026/eb1641a6/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: test.ll
Type: application/octet-stream
Size: 3962 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181026/eb1641a6/attachment.obj>
More information about the llvm-dev
mailing list