Carve out a place in instcombine to put transformations which work knowing that their
result is non-zero. Implement an example optimization (PR9814), which allows us to
transform:
A / ((1 << B) >>u 2)
into:
A >>u (B-2)
which we compile into:
_divu3: ## @divu3
leal -2(%rsi), %ecx
shrl %cl, %edi
movl %edi, %eax
ret
instead of:
_divu3: ## @divu3
movb %sil, %cl
movl $1, %esi
shll %cl, %esi
shrl $2, %esi
movl %edi, %eax
xorl %edx, %edx
divl %esi, %eax
ret
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131860 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index c4dee6a..c96741a 100644
--- a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -19,6 +19,31 @@
using namespace llvm;
using namespace PatternMatch;
+
+/// simplifyValueKnownNonZero - The specific integer value is used in a context
+/// where it is known to be non-zero. If this allows us to simplify the
+/// computation, do so and return the new operand, otherwise return null.
+static Value *simplifyValueKnownNonZero(Value *V, InstCombiner &IC) {
+ // If V has multiple uses, then we would have to do more analysis to determine
+ // if this is safe. For example, the use could be in dynamically unreached
+ // code.
+ if (!V->hasOneUse()) return 0;
+
+ // ((1 << A) >>u B) --> (1 << (A-B))
+ // Because V cannot be zero, we know that B is less than A.
+ Value *A = 0, *B = 0; ConstantInt *One = 0;
+ if (match(V, m_LShr(m_OneUse(m_Shl(m_ConstantInt(One), m_Value(A))),
+ m_Value(B))) &&
+ // The "1" can be any value known to be a power of 2.
+ One->getValue().isPowerOf2()) {
+ A = IC.Builder->CreateSub(A, B, "tmp");
+ return IC.Builder->CreateShl(One, A);
+ }
+
+ return 0;
+}
+
+
/// MultiplyOverflows - True if the multiply can not be expressed in an int
/// this size.
static bool MultiplyOverflows(ConstantInt *C1, ConstantInt *C2, bool sign) {
@@ -293,6 +318,12 @@
Instruction *InstCombiner::commonIDivTransforms(BinaryOperator &I) {
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
+ // The RHS is known non-zero.
+ if (Value *V = simplifyValueKnownNonZero(I.getOperand(1), *this)) {
+ I.setOperand(1, V);
+ return &I;
+ }
+
// Handle cases involving: [su]div X, (select Cond, Y, Z)
// This does not apply for fdiv.
if (isa<SelectInst>(Op1) && SimplifyDivRemOfSelect(I))
@@ -499,6 +530,12 @@
Instruction *InstCombiner::commonIRemTransforms(BinaryOperator &I) {
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
+ // The RHS is known non-zero.
+ if (Value *V = simplifyValueKnownNonZero(I.getOperand(1), *this)) {
+ I.setOperand(1, V);
+ return &I;
+ }
+
// Handle cases involving: rem X, (select Cond, Y, Z)
if (isa<SelectInst>(Op1) && SimplifyDivRemOfSelect(I))
return &I;