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Simplify (X<<1)-X into X. According to my auto-simplier this is the most common missed

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simplification in fully optimized code.  It occurs sporadically in the testsuite, and
many times in 403.gcc: the final bitcode has 131 fewer subtractions after this change.
The reason that the multiplies are not eliminated is the same reason that instcombine
did not catch this: they are used by other instructions (instcombine catches this with
a more general transform which in general is only profitable if the operands have only
one use).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123754 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Duncan Sands 2011-01-18 09:24:58 +00:00
parent 908d8e82b5
commit fe02c69f84
2 changed files with 26 additions and 0 deletions
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6
lib/Analysis/InstructionSimplify.cpp
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@ -593,6 +593,12 @@ static Value *SimplifySubInst(Value *Op0, Value *Op1, bool isNSW, bool isNUW,
match(Op0, m_Add(m_Specific(Op1), m_Value(X))))
return X;
// (X*2) - X -> X
// (X<<1) - X -> X
if (match(Op0, m_Mul(m_Specific(Op1), m_ConstantInt<2>())) ||
match(Op0, m_Shl(m_Specific(Op1), m_One())))
return Op1;
// i1 sub -> xor.
if (MaxRecurse && Op0->getType()->isIntegerTy(1))
if (Value *V = SimplifyXorInst(Op0, Op1, TD, DT, MaxRecurse-1))

20
test/Transforms/InstSimplify/2010-12-20-Distribute.ll
View File

@ -31,6 +31,26 @@ define i32 @factorize3(i32 %x, i32 %a, i32 %b) {
; CHECK: ret i32 %r
}
define i32 @factorize4(i32 %x, i32 %y) {
; CHECK: @factorize4
%sh = shl i32 %y, 1
%ml = mul i32 %sh, %x
%mr = mul i32 %x, %y
%s = sub i32 %ml, %mr
ret i32 %s
; CHECK: ret i32 %mr
}
define i32 @factorize5(i32 %x, i32 %y) {
; CHECK: @factorize5
%sh = mul i32 %y, 2
%ml = mul i32 %sh, %x
%mr = mul i32 %x, %y
%s = sub i32 %ml, %mr
ret i32 %s
; CHECK: ret i32 %mr
}
define i32 @expand(i32 %x) {
; CHECK: @expand
; ((X & 1) | 2) & 1 -> ((X & 1) & 1) | (2 & 1) -> (X & 1) | 0 -> X & 1