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[InstCombine] Simplify (x >> y) <= x

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This commit extends the patterns recognised by InstSimplify to also handle (x >> y) <= x in the same way as (x /u y) <= x.

The missing optimisation was found investigating why LLVM did not optimise away bound checks in a binary search: https://github.com/rust-lang/rust/pull/30917

Patch by Andrea Canciani!

Differential Revision: http://reviews.llvm.org/D16402

llvm-svn: 258422
This commit is contained in:
David Majnemer 2016-01-21 18:55:54 +00:00
parent b9cedfb407
commit 4981d2326a
2 changed files with 20 additions and 2 deletions
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6
lib/Analysis/InstructionSimplify.cpp
View File

@ -2727,9 +2727,11 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
}
}
// x >> y <=u x
// x udiv y <=u x.
if (LBO && match(LBO, m_UDiv(m_Specific(RHS), m_Value()))) {
// icmp pred (X /u Y), X
if (LBO && (match(LBO, m_LShr(m_Specific(RHS), m_Value())) ||
match(LBO, m_UDiv(m_Specific(RHS), m_Value())))) {
// icmp pred (X op Y), X
if (Pred == ICmpInst::ICMP_UGT)
return getFalse(ITy);
if (Pred == ICmpInst::ICMP_ULE)

16
test/Transforms/InstSimplify/compare.ll
View File

@ -397,6 +397,22 @@ define i1 @lshr3(i32 %x) {
; CHECK: ret i1 true
}
define i1 @lshr4(i32 %X, i32 %Y) {
; CHECK-LABEL: @lshr4(
%A = lshr i32 %X, %Y
%C = icmp ule i32 %A, %X
ret i1 %C
; CHECK: ret i1 true
}
define i1 @lshr5(i32 %X, i32 %Y) {
; CHECK-LABEL: @lshr5(
%A = lshr i32 %X, %Y
%C = icmp ugt i32 %A, %X
ret i1 %C
; CHECK: ret i1 false
}
define i1 @ashr1(i32 %x) {
; CHECK-LABEL: @ashr1(
%s = ashr i32 -1, %x