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[ScalarEvolution] Fix PR24757.

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Summary:
PR24757 was caused by some incorect math in
`ScalarEvolution::HowFarToZero` -- the smallest unsigned solution for X
in

  2^N * A = 2^N * X

is not necessarily A.

Reviewers: atrick, majnemer, meheff

Subscribers: llvm-commits, sanjoy

Differential Revision: http://reviews.llvm.org/D12721

llvm-svn: 247242
This commit is contained in:
Sanjoy Das 2015-09-10 05:27:38 +00:00
parent 812097ec2e
commit 33ef65354e
2 changed files with 77 additions and 2 deletions
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44
lib/Analysis/ScalarEvolution.cpp
View File

@ -6306,8 +6306,48 @@ ScalarEvolution::HowFarToZero(const SCEV *V, const Loop *L, bool ControlsExit) {
// also returns true if StepV is maximally negative (eg, INT_MIN), but that
// case is not handled as this code is guarded by !CountDown.
if (StepV.isPowerOf2() &&
GetMinTrailingZeros(Distance) >= StepV.countTrailingZeros())
return getUDivExactExpr(Distance, Step);
GetMinTrailingZeros(Distance) >= StepV.countTrailingZeros()) {
// Here we've constrained the equation to be of the form
//
// 2^(N + k) * Distance' = (StepV == 2^N) * X (mod 2^W) ... (0)
//
// where we're operating on a W bit wide integer domain and k is
// non-negative. The smallest unsigned solution for X is the trip count.
//
// (0) is equivalent to:
//
// 2^(N + k) * Distance' - 2^N * X = L * 2^W
// <=> 2^N(2^k * Distance' - X) = L * 2^(W - N) * 2^N
// <=> 2^k * Distance' - X = L * 2^(W - N)
// <=> 2^k * Distance' = L * 2^(W - N) + X ... (1)
//
// The smallest X satisfying (1) is unsigned remainder of dividing the LHS
// by 2^(W - N).
//
// <=> X = 2^k * Distance' URem 2^(W - N) ... (2)
//
// E.g. say we're solving
//
// 2 * Val = 2 * X (in i8) ... (3)
//
// then from (2), we get X = Val URem i8 128 (k = 0 in this case).
//
// Note: It is tempting to solve (3) by setting X = Val, but Val is not
// necessarily the smallest unsigned value of X that satisfies (3).
// E.g. if Val is i8 -127 then the smallest value of X that satisfies (3)
// is i8 1, not i8 -127
const auto *ModuloResult = getUDivExactExpr(Distance, Step);
// Since SCEV does not have a URem node, we construct one using a truncate
// and a zero extend.
unsigned NarrowWidth = StepV.getBitWidth() - StepV.countTrailingZeros();
auto *NarrowTy = IntegerType::get(getContext(), NarrowWidth);
auto *WideTy = Distance->getType();
return getZeroExtendExpr(getTruncateExpr(ModuloResult, NarrowTy), WideTy);
}
}
// If the condition controls loop exit (the loop exits only if the expression

35
test/Analysis/ScalarEvolution/pr24757.ll Normal file
View File

@ -0,0 +1,35 @@
; RUN: opt -analyze -scalar-evolution < %s | FileCheck %s
; CHECK: Loop %bb1: backedge-taken count is (zext i7 (trunc i8 %a.promoted to i7) to i8)
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.10.0"
@a = global i8 -127, align 1
@b = common global i32 0, align 4
declare void @use(i32)
define i32 @main() {
bb:
%a.promoted = load i8, i8* @a
br label %bb1
bb1: ; preds = %bb1, %bb
%tmp = phi i8 [ %tmp2, %bb1 ], [ %a.promoted, %bb ]
%tmp2 = add i8 %tmp, -1
%tmp3 = sext i8 %tmp to i32
%tmp4 = xor i32 %tmp3, -1
%tmp5 = sext i8 %tmp2 to i32
%tmpf = sub nsw i32 %tmp4, %tmp5
%tmp6 = trunc i32 %tmpf to i8
%tmp7 = icmp eq i8 %tmp6, 0
br i1 %tmp7, label %bb8, label %bb1
bb8: ; preds = %bb1
store i8 %tmp2, i8* @a
store i32 %tmp4, i32* @b
%tmp9 = sext i8 %tmp2 to i32
call void @use(i32 %tmp9)
ret i32 0
}