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aba875a2e8
This patch updates ValueLattice to distinguish between ranges that are
guaranteed to not include undef and ranges that may include undef.
A constant range guaranteed to not contain undef can be used to simplify
instructions to arbitrary values. A constant range that may contain
undef can only be used to simplify to a constant. If the value can be
undef, it might take a value outside the range. For example, consider
the snipped below
define i32 @f(i32 %a, i1 %c) {
br i1 %c, label %true, label %false
true:
%a.255 = and i32 %a, 255
br label %exit
false:
br label %exit
exit:
%p = phi i32 [ %a.255, %true ], [ undef, %false ]
%f.1 = icmp eq i32 %p, 300
call void @use(i1 %f.1)
%res = and i32 %p, 255
ret i32 %res
}
In the exit block, %p would be a constant range [0, 256) including undef as
%p could be undef. We can use the range information to replace %f.1 with
false because we remove the compare, effectively forcing the use of the
constant to be != 300. We cannot replace %res with %p however, because
if %a would be undef %cond may be true but the second use might not be
< 256.
Currently LazyValueInfo uses the new behavior just when simplifying AND
instructions and does not distinguish between constant ranges with and
without undef otherwise. I think we should address the remaining issues
in LVI incrementally.
Reviewers: efriedma, reames, aqjune, jdoerfert, sstefan1
Reviewed By: efriedma
Differential Revision: https://reviews.llvm.org/D76931
34 lines
823 B
LLVM
34 lines
823 B
LLVM
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
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; RUN: opt -ipsccp -S %s | FileCheck %s
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declare i1 @cond()
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define i32 @test() {
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; CHECK-LABEL: @test(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: br label [[LOOP:%.*]]
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; CHECK: loop:
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; CHECK-NEXT: [[C_1:%.*]] = call i1 @cond()
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; CHECK-NEXT: br i1 [[C_1]], label [[LOOP]], label [[LATCH_2:%.*]]
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; CHECK: latch.2:
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; CHECK-NEXT: [[C_2:%.*]] = call i1 @cond()
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; CHECK-NEXT: br i1 [[C_2]], label [[LOOP]], label [[EXIT:%.*]]
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; CHECK: exit:
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; CHECK-NEXT: ret i32 0
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;
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entry:
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br label %loop
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loop:
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%p = phi i32 [ undef, %entry ], [ 0, %latch.2 ], [ %p, %loop]
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%c.1 = call i1 @cond()
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br i1 %c.1, label %loop, label %latch.2
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latch.2:
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%c.2 = call i1 @cond()
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br i1 %c.2, label %loop, label %exit
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exit:
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ret i32 %p
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}
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