IRCE wouldn't deal with range checks which predicate is uge.


Bugzilla Link	49012
Version	11.0
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@preames

Extended Description

recently, I tried to use llvm to optimize my code generated by a managed VM. this VM will instrument many range check code 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 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

llvm / llvm-project

IRCE wouldn't deal with range checks which predicate is uge. #48356

Extended Description