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19453c8511
Summary:
Specifically, when we perform runtime loop unrolling of a loop that
contains a convergent op, we can only unroll k times, where k divides
the loop trip multiple.
Without this change, we'll happily unroll e.g. the following loop
for (int i = 0; i < N; ++i) {
if (i == 0) convergent_op();
foo();
}
into
int i = 0;
if (N % 2 == 1) {
convergent_op();
foo();
++i;
}
for (; i < N - 1; i += 2) {
if (i == 0) convergent_op();
foo();
foo();
}.
This is unsafe, because we've just added a control-flow dependency to
the convergent op in the prelude.
In general, runtime unrolling loops that contain convergent ops is safe
only if we don't have emit a prelude, which occurs when the unroll count
divides the trip multiple.
Reviewers: resistor
Subscribers: llvm-commits, mzolotukhin
Differential Revision: http://reviews.llvm.org/D17526
llvm-svn: 263509
84 lines
2.0 KiB
LLVM
84 lines
2.0 KiB
LLVM
; RUN: opt < %s -loop-unroll -unroll-runtime -unroll-allow-partial -S | FileCheck %s
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declare void @f() convergent
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; Although this loop contains a convergent instruction, it should be
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; fully unrolled.
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;
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; CHECK-LABEL: @full_unroll(
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define i32 @full_unroll() {
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entry:
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br label %l3
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l3:
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%x.0 = phi i32 [ 0, %entry ], [ %inc, %l3 ]
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; CHECK: call void @f()
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; CHECK: call void @f()
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; CHECK: call void @f()
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; CHECK-NOT: call void @f()
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call void @f() ;convergent
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%inc = add nsw i32 %x.0, 1
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%exitcond = icmp eq i32 %inc, 3
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br i1 %exitcond, label %exit, label %l3
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exit:
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ret i32 0
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}
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; This loop contains a convergent instruction, but it should be partially
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; unrolled. The unroll count is the largest power of 2 that divides the
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; multiple -- 4, in this case.
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;
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; CHECK-LABEL: @runtime_unroll(
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define i32 @runtime_unroll(i32 %n) {
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entry:
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%loop_ctl = mul nsw i32 %n, 12
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br label %l3
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l3:
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%x.0 = phi i32 [ 0, %entry ], [ %inc, %l3 ]
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; CHECK: call void @f()
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; CHECK: call void @f()
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; CHECK: call void @f()
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; CHECK: call void @f()
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; CHECK-NOT: call void @f()
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call void @f() convergent
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%inc = add nsw i32 %x.0, 1
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%exitcond = icmp eq i32 %inc, %loop_ctl
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br i1 %exitcond, label %exit, label %l3
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exit:
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ret i32 0
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}
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; This loop contains a convergent instruction, so its partial unroll
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; count must divide its trip multiple. This overrides its unroll
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; pragma -- we unroll exactly 8 times, even though 16 is requested.
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; CHECK-LABEL: @pragma_unroll
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define i32 @pragma_unroll(i32 %n) {
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entry:
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%loop_ctl = mul nsw i32 %n, 24
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br label %l3, !llvm.loop !0
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l3:
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%x.0 = phi i32 [ 0, %entry ], [ %inc, %l3 ]
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; CHECK: call void @f()
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; CHECK: call void @f()
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; CHECK: call void @f()
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; CHECK: call void @f()
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; CHECK: call void @f()
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; CHECK: call void @f()
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; CHECK: call void @f()
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; CHECK: call void @f()
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; CHECK-NOT: call void @f()
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call void @f() convergent
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%inc = add nsw i32 %x.0, 1
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%exitcond = icmp eq i32 %inc, %loop_ctl
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br i1 %exitcond, label %exit, label %l3, !llvm.loop !0
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exit:
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ret i32 0
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}
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!0 = !{!0, !{!"llvm.loop.unroll.count", i32 16}}
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