[llvm-dev] 回复: [RFC] Improve iteration of estimating divisions

Qiu Chaofan via llvm-dev llvm-dev at lists.llvm.org
Thu Aug 8 09:47:03 PDT 2019


Yes, speed is an important factor of making dicision. Here I just put the numerator into estimation, so it won't add any more instructions. A simple benchmark below keeps the same running time between the demo and current master:

float fdiv(unsigned int a, unsigned int b) {
  return (float)a / (float)b;

float m;

__attribute__((noinline)) void foo() {
  m = 0.0;

int main() {
  for (int i = 1; i < 1000000; ++i)
    for (int j = 1; j < 30000; ++j) {
      m = fdiv(i, j);

Qiu Chaofan

发件人: Finkel, Hal J. <hfinkel at anl.gov>
发送时间: 2019年8月7日 4:04
收件人: 邱 超凡; llvm-dev at lists.llvm.org
主题: Re: [llvm-dev] [RFC] Improve iteration of estimating divisions

On 8/6/19 12:20 AM, 邱 超凡 via llvm-dev wrote:
> Hi there, I notice that our current implementation of fast division transformation (turn `a / b` into `a * (1/b)`) is worse in precision compared with GCC.  Like this case in ppc64le:
>          float fdiv(unsigned int a, unsigned int b) {
>                  return (float)a / (float)b;
>          }
> Result of Clang -Ofast is 41A00001 (in Hex), while GCC produces 41A00000 which is the same as no optimizations opened.
> Currently, DAGCombiner uses `BuildReciprocalEstimate` to calculate the reciprocal (`1/b`) first and multiply it with `a`.  But if we put the operand `a` into iterations in the estimate function, the result would be better.
> Patching such a change may break several existing test cases in different platforms since it’s target-independent code.  So any suggestions are welcome.  Thanks.

Test cases can be changed if the result is universally better, and
alternatively, we can introduce a way for the target to control the
behavior (e.g., how we choose between buildSqrtNROneConst and
buildSqrtNRTwoConst). What's the effect on performance?


> Regards,
> Qiu Chaofan
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Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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