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[ValueTracking] Revert r310583 which enabled functionality that still is 

causing compile time issues.

Moreover, the patch *deleted* the flag in addition to changing the
default, and links to a code review that doesn't even discuss the flag
and just has an update to a Clang test case.

I've followed up on the commit thread to ask for numbers on compile time
at this point, leaving the flag in place until things stabilize, and
pointing at specific code that seems to exhibit excessive compile time
with this patch.

Original commit message for r310583:
"""
[ValueTracking] Enabling ValueTracking patch by default (recommit). Part 2.

The original patch was an improvement to IR ValueTracking on
non-negative integers. It has been checked in to trunk (D18777,
r284022). But was disabled by default due to performance regressions.
Perf impact has improved. The patch would be enabled by default.
""""

llvm-svn: 310816
This commit is contained in:
Chandler Carruth 2017-08-14 07:03:24 +00:00
parent 1e09c1363c
commit 37c7b08710
2 changed files with 9 additions and 27 deletions
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9
llvm/lib/Analysis/ValueTracking.cpp
View File

@ -54,6 +54,12 @@ const unsigned MaxDepth = 6;
static cl::opt<unsigned> DomConditionsMaxUses("dom-conditions-max-uses",
cl::Hidden, cl::init(20));
// This optimization is known to cause performance regressions is some cases,
// keep it under a temporary flag for now.
static cl::opt<bool>
DontImproveNonNegativePhiBits("dont-improve-non-negative-phi-bits",
cl::Hidden, cl::init(true));
/// Returns the bitwidth of the given scalar or pointer type. For vector types,
/// returns the element type's bitwidth.
static unsigned getBitWidth(Type *Ty, const DataLayout &DL) {
@ -1252,6 +1258,9 @@ static void computeKnownBitsFromOperator(const Operator *I, KnownBits &Known,
Known.Zero.setLowBits(std::min(Known2.countMinTrailingZeros(),
Known3.countMinTrailingZeros()));
if (DontImproveNonNegativePhiBits)
break;
auto *OverflowOp = dyn_cast<OverflowingBinaryOperator>(LU);
if (OverflowOp && OverflowOp->hasNoSignedWrap()) {
// If initial value of recurrence is nonnegative, and we are adding

27
llvm/test/Analysis/ValueTracking/non-negative-phi-bits.ll
View File

@ -1,27 +0,0 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt -instcombine < %s -S | FileCheck %s
define void @test() #0 {
; CHECK-LABEL: @test(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[FOR_BODY:%.*]]
; CHECK: for.body:
; CHECK-NEXT: [[INDVARS_IV:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[INDVARS_IV_NEXT:%.*]], [[FOR_BODY]] ]
; CHECK-NEXT: [[INDVARS_IV_NEXT]] = add nuw nsw i64 [[INDVARS_IV]], 1
; CHECK-NEXT: [[EXITCOND:%.*]] = icmp ult i64 [[INDVARS_IV_NEXT]], 40
; CHECK-NEXT: br i1 [[EXITCOND]], label [[FOR_END:%.*]], label [[FOR_BODY]]
; CHECK: for.end:
; CHECK-NEXT: ret void
;
entry:
br label %for.body
for.body: ; preds = %for.body, %entry
%indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
%indvars.iv.next = add nsw i64 %indvars.iv, 1
%exitcond = icmp slt i64 %indvars.iv.next, 40
br i1 %exitcond, label %for.end, label %for.body
for.end: ; preds = %for.body
ret void
}