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[PPC/LoopUnrollRuntime] Don't avoid high-cost trip count computation on the PPC/A2

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On X86 (and similar OOO cores) unrolling is very limited, and even if the
runtime unrolling is otherwise profitable, the expense of a division to compute
the trip count could greatly outweigh the benefits. On the A2, we unroll a lot,
and the benefits of unrolling are more significant (seeing a 5x or 6x speedup
is not uncommon), so we're more able to tolerate the expense, on average, of a
division to compute the trip count.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237947 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Hal Finkel 2015-05-21 20:30:23 +00:00
parent 3b294b5753
commit 5368a26e16
2 changed files with 31 additions and 0 deletions
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4
lib/Target/PowerPC/PPCTargetTransformInfo.cpp
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@ -187,6 +187,10 @@ void PPCTTIImpl::getUnrollingPreferences(Loop *L,
// The A2 is in-order with a deep pipeline, and concatenation unrolling
// helps expose latency-hiding opportunities to the instruction scheduler.
UP.Partial = UP.Runtime = true;
// We unroll a lot on the A2 (hundreds of instructions), and the benefits
// often outweigh the cost of a division to compute the trip count.
UP.AllowExpensiveTripCount = true;
}
BaseT::getUnrollingPreferences(L, UP);

27
test/Transforms/LoopUnroll/PowerPC/a2-high-cost-trip-count-computation.ll Normal file
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@ -0,0 +1,27 @@
; RUN: opt < %s -S -mtriple=powerpc64-unknown-linux-gnu -mcpu=a2 -loop-unroll | FileCheck %s
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
;; Check that we do emit expensive instructions to compute trip
;; counts when unrolling loops on the a2 (because we unroll a lot).
define i32 @test(i64 %v12, i8* %array, i64* %loc) {
; CHECK-LABEL: @test(
; CHECK: udiv
entry:
%step = load i64, i64* %loc, !range !0
br label %loop
loop: ; preds = %entry, %loop
%k.015 = phi i64 [ %v15, %loop ], [ %v12, %entry ]
%v14 = getelementptr inbounds i8, i8* %array, i64 %k.015
store i8 0, i8* %v14
%v15 = add nuw nsw i64 %k.015, %step
%v16 = icmp slt i64 %v15, 8193
br i1 %v16, label %loop, label %loopexit
loopexit: ; preds = %loop
ret i32 0
}
!0 = !{i64 1, i64 100}