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

[Jie He via llvm-dev]

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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