[llvm-dev] SCEV and (original) (raw)

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

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}<%loop.body> U: [0,2147483647) S: [0,2147483647) Exits: (-1 + %x) LoopDispositions: { %loop.body: Computable } ; %i.next = add nsw i32 %i, 1 ; --> {1,+,1}<%loop.body> U: [1,-2147483648) S: [1,-2147483648) Exits: %x LoopDispositions: { %loop.body: Computable } ; %i.96 = add nsw i32 %i, 96 ; --> {96,+,1}<%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

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

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}<%loop.body> U: [0,2147483647) S: [0,2147483647) Exits: (-1 + %x) LoopDispositions: { %loop.body: Computable } ; %i.next = add nsw i32 %i, 1 ; --> {1,+,1}<%loop.body> U: [1,-2147483648) S: [1,-2147483648) Exits: %x LoopDispositions: { %loop.body: Computable } ; %i.96 = add nsw i32 %i, 96 ; --> {96,+,1}<%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}<%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}<%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}<%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 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}<%loop.body> U: [0,2147483647) S: [0,2147483647) Exits: (-1 + %x) LoopDispositions: { %loop.body: Computable } %i.next = add nsw i32 %i, 1 --> {1,+,1}<%loop.body> U: [1,-2147483648) S: [1,-2147483648) Exits: %x LoopDispositions: { %loop.body: Computable } %i.96 = add nsw i32 %i, 96 --> {96,+,1}<%loop.body> U: [96,-2147483648) S: [96,-2147483648) Exits: (95 + %x) LoopDispositions: { %loop.body: Computable } Help? -- Alexandre Isoard

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