[InstructionSimplify] Add support for saturating add/sub

Add support for saturating add/sub in InstructionSimplify. In particular, the following simplifications are supported:

    sat(X + 0) -> X
    sat(X + undef) -> -1
    sat(X uadd MAX) -> MAX
    (and commutative variants)

    sat(X - 0) -> X
    sat(X - X) -> 0
    sat(X - undef) -> 0
    sat(undef - X) -> 0
    sat(0 usub X) -> 0
    sat(X usub MAX) -> 0

Patch by: @nikic (Nikita Popov)

Differential Revision: https://reviews.llvm.org/D54532

llvm-svn: 347330

1 files changed, 34 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 0039d1eebc0..599a2f2ce4c 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4912,6 +4912,40 @@ static Value *simplifyBinaryIntrinsic(Function *F, Value *Op0, Value *Op1,
     if (match(Op0, m_Undef()) || match(Op1, m_Undef()))
       return Constant::getNullValue(ReturnType);
     break;
+  case Intrinsic::uadd_sat:
+    // sat(MAX + X) -> MAX
+    // sat(X + MAX) -> MAX
+    if (match(Op0, m_AllOnes()) || match(Op1, m_AllOnes()))
+      return Constant::getAllOnesValue(ReturnType);
+    LLVM_FALLTHROUGH;
+  case Intrinsic::sadd_sat:
+    // sat(X + undef) -> -1
+    // sat(undef + X) -> -1
+    // For unsigned: Assume undef is MAX, thus we saturate to MAX (-1).
+    // For signed: Assume undef is ~X, in which case X + ~X = -1.
+    if (match(Op0, m_Undef()) || match(Op1, m_Undef()))
+      return Constant::getAllOnesValue(ReturnType);
+
+    // X + 0 -> X
+    if (match(Op1, m_Zero()))
+      return Op0;
+    // 0 + X -> X
+    if (match(Op0, m_Zero()))
+      return Op1;
+    break;
+  case Intrinsic::usub_sat:
+    // sat(0 - X) -> 0, sat(X - MAX) -> 0
+    if (match(Op0, m_Zero()) || match(Op1, m_AllOnes()))
+      return Constant::getNullValue(ReturnType);
+    LLVM_FALLTHROUGH;
+  case Intrinsic::ssub_sat:
+    // X - X -> 0, X - undef -> 0, undef - X -> 0
+    if (Op0 == Op1 || match(Op0, m_Undef()) || match(Op1, m_Undef()))
+      return Constant::getNullValue(ReturnType);
+    // X - 0 -> X
+    if (match(Op1, m_Zero()))
+      return Op0;
+    break;
   case Intrinsic::load_relative:
     if (auto *C0 = dyn_cast<Constant>(Op0))
       if (auto *C1 = dyn_cast<Constant>(Op1))