[llvm-dev] Why IRCE wouldn't deal with the range check which has a predicate "uge"?

[Philip Reames via llvm-dev]

)Quick response, may be off base as I'm taking a guess based on a quick 
read through.)

IRCE - like many llvm passes - assumes IR has been canonicalized by 
other passes.  If you run your IR through -O3 and then IRCE, do you get 
what you expect?

Philip

On 2/4/21 12:43 AM, Jie He via llvm-dev wrote:
> Hi
> Recently, I tried to use llvm to optimize my code generated by a managed VM. this VM will instrument many range check codes before memory access operations.
> I found the optimization IRCE wouldn't work when the range check with the predicate "uge". I wrote the following c/c++ code to simulate my code, where len is the max buf length:
>
> void testIRCE(unsigned int * buf, unsigned int len, unsigned int iteration_count) {
>      if (iteration_count > 0) {
>          unsigned int i = 0;
>          do {
>              if (i >= len) { // range check
>                  printf("overflow\n");
>                  return;
>              }
>
>              buf[i] = i;
>
>              i ++;
>
>          } while (i < iteration_count);
>      }
> }
>
> the above code wouldn't be optimised as expected into 2 loops (iteration range splitting). I checked the llvm code, found in the function InductiveRangeCheck::parseRangeCheckICmp(), it wouldn't deal with uge cases, I'm not sure if it is intentional.
>
> but I tried to modify the llvm IR code by replacing the uge to ult, and interchanging the operands of next branch instruction. the optimization IRCE works as expected.
>
> my test command is below:
> first, get a clean llvm IR file.
> ./clang++ -O3 -Xclang -disable-llvm-passes  ~/testRCE.cpp  -emit-llvm -S -o ~/testRCE.TBBA.ll
>
> second, optimize it with other loop optimization.
> ./opt -gvn -simplifycfg -loop-simplify -loop-predication -licm -dce -mem2reg -dce -jump-threading -lcssa -simplifycfg -loop-simplify  -dce -stats -debug-pass=Executions ~/testRCE.TBBA.ll -S -o ~/testRCE.mem2reg.ll
>
> finally, take IRCE.
> ./opt -irce-skip-profitability-checks -irce -dce -S 
> ~/testRCE.mem2reg.ll -o ~/testRCE.irce.ll--
>
>
> Best Regards
> He Jie 何杰
>
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