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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
45 lines
1.2 KiB
LLVM
45 lines
1.2 KiB
LLVM
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
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; RUN: opt -S -ipsccp %s | FileCheck %s
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; Make sure IPSCCP does not assume %r < 256 for @f1. Undef is passed at a call
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; site, which won't be eliminated.
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define i1 @constant_and_undef(i64 %a) {
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; CHECK-LABEL: @constant_and_undef(
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; CHECK-NEXT: [[C_1:%.*]] = call i1 @f1(i64 undef)
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; CHECK-NEXT: br label [[BB1:%.*]]
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; CHECK: bb1:
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; CHECK-NEXT: [[C_2:%.*]] = call i1 @f1(i64 10)
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; CHECK-NEXT: br label [[BB2:%.*]]
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; CHECK: bb2:
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; CHECK-NEXT: [[RANGE:%.*]] = and i64 [[A:%.*]], 255
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; CHECK-NEXT: [[C_3:%.*]] = call i1 @f1(i64 [[RANGE]])
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; CHECK-NEXT: ret i1 true
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;
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%c.1 = call i1 @f1(i64 undef)
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br label %bb1
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bb1:
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%c.2 = call i1 @f1(i64 10)
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br label %bb2
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bb2:
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%range = and i64 %a, 255
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%c.3 = call i1 @f1(i64 %range)
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%r.1 = and i1 %c.1, %c.2
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%r.2 = and i1 %r.1, %c.3
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ret i1 %r.2
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}
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declare void @sideeffect(i1, i64 %a)
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define internal i1 @f1(i64 %r) {
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; CHECK-LABEL: define {{.*}} @f1(
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; CHECK-NEXT: call void @sideeffect(i1 true, i64 [[R:%.*]])
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; CHECK-NEXT: ret i1 undef
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;
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%c = icmp ult i64 %r, 256
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call void @sideeffect(i1 %c, i64 %r)
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ret i1 %c
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}
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