Is there a simple way to check APInt multiplication overflow?

benshi001 · July 11, 2020, 11:48am

Hello,

I have two APInt variables, which are got by ConstantSDNode::getAPIntValue().

Is there a simple way to know if their multiplication overflows?

For example, if both APInt vars contain a int32_t,
APInt a, b;
Then I think

if (a.getMinSignedBits() + b.getMinSignedBits() > 32)
a*b must exceeds int32_t <=> overflow happens;

Is my way OK?

Ben Shi

LebedevRI · July 11, 2020, 11:53am

Hello,

I have two APInt variables, which are got by ConstantSDNode::getAPIntValue().

Is there a simple way to know if their multiplication overflows?

There's APInt::smul_ov(const APInt &RHS, bool &Overflow)

github.com

llvm/llvm-project/blob/d7a05698efcfa6c596bcaadd8d5154612990f8f3/llvm/include/llvm/ADT/APInt.h#L1115-L1134


      
          // Operations that return overflow indicators.
          APInt sadd_ov(const APInt &RHS, bool &Overflow) const;
          APInt uadd_ov(const APInt &RHS, bool &Overflow) const;
          APInt ssub_ov(const APInt &RHS, bool &Overflow) const;
          APInt usub_ov(const APInt &RHS, bool &Overflow) const;
          APInt sdiv_ov(const APInt &RHS, bool &Overflow) const;
          APInt smul_ov(const APInt &RHS, bool &Overflow) const;
          APInt umul_ov(const APInt &RHS, bool &Overflow) const;
          APInt sshl_ov(const APInt &Amt, bool &Overflow) const;
          APInt ushl_ov(const APInt &Amt, bool &Overflow) const;
          
          // Operations that saturate
          APInt sadd_sat(const APInt &RHS) const;
          APInt uadd_sat(const APInt &RHS) const;
          APInt ssub_sat(const APInt &RHS) const;
          APInt usub_sat(const APInt &RHS) const;
          APInt smul_sat(const APInt &RHS) const;
          APInt umul_sat(const APInt &RHS) const;
          APInt sshl_sat(const APInt &RHS) const;
          APInt ushl_sat(const APInt &RHS) const;

For example, if both APInt vars contain a int32_t,
APInt a, b;
Then I think

if (a.getMinSignedBits() + b.getMinSignedBits() > 32)
a*b must exceeds int32_t <=> overflow happens;

Is my way OK?

Ben Shi

Roman