InstCombine: Turn (zext A) udiv (zext B) into (zext (A udiv B)). Same for urem or constant B.

This obviously helps a lot if the division would be turned into a libcall
(think i64 udiv on i386), but div is also one of the few remaining instructions
on modern CPUs that become more expensive when the bitwidth gets bigger.

This also helps register pressure on i386 when dividing chars, divb needs
two 8-bit parts of a 16 bit register as input where divl uses two registers.

int foo(unsigned char a) { return a/10; }
int bar(unsigned char a, unsigned char b) { return a/b; }

compiles into (x86_64)
_foo:
  imull $205, %edi, %eax
  shrl  $11, %eax
  ret
_bar:
  movzbl        %dil, %eax
  divb  %sil, %al
  movzbl        %al, %eax
  ret

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130615 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 28 insertions, 1 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index b4f4da6..230b80e 100644
--- a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -336,6 +336,19 @@ Instruction *InstCombiner::commonIDivTransforms(BinaryOperator &I) {
   return 0;
 }
 
+/// dyn_castZExtVal - Checks if V is a zext or constant that can
+/// be truncated to Ty without losing bits.
+static Value *dyn_castZExtVal(Value *V, const Type *Ty) {
+  if (ZExtInst *Z = dyn_cast<ZExtInst>(V)) {
+    if (Z->getSrcTy() == Ty)
+      return Z->getOperand(0);
+  } else if (ConstantInt *C = dyn_cast<ConstantInt>(V)) {
+    if (C->getValue().getActiveBits() <= cast<IntegerType>(Ty)->getBitWidth())
+      return ConstantExpr::getTrunc(C, Ty);
+  }
+  return 0;
+}
+
 Instruction *InstCombiner::visitUDiv(BinaryOperator &I) {
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
 
@@ -394,6 +407,14 @@ Instruction *InstCombiner::visitUDiv(BinaryOperator &I) {
       return SelectInst::Create(Cond, TSI, FSI);
     }
   }
+
+  // (zext A) udiv (zext B) --> zext (A udiv B)
+  if (ZExtInst *ZOp0 = dyn_cast<ZExtInst>(Op0))
+    if (Value *ZOp1 = dyn_castZExtVal(Op1, ZOp0->getSrcTy()))
+      return new ZExtInst(Builder->CreateUDiv(ZOp0->getOperand(0), ZOp1, "div",
+                                              I.isExact()),
+                          I.getType());
+
   return 0;
 }
 
@@ -568,7 +589,13 @@ Instruction *InstCombiner::visitURem(BinaryOperator &I) {
       return SelectInst::Create(Cond, TrueAnd, FalseAnd);
     }
   }
-  
+
+  // (zext A) urem (zext B) --> zext (A urem B)
+  if (ZExtInst *ZOp0 = dyn_cast<ZExtInst>(Op0))
+    if (Value *ZOp1 = dyn_castZExtVal(Op1, ZOp0->getSrcTy()))
+      return new ZExtInst(Builder->CreateURem(ZOp0->getOperand(0), ZOp1),
+                          I.getType());
+
   return 0;
 }