[llvm-dev] IRBuilder constraints

Norbert Dajka via llvm-dev llvm-dev at lists.llvm.org
Tue Feb 5 01:41:49 PST 2019


I need to control IRBuilder(or manage IR building with other means) very
strictly. The problem is in the realm of ConstInt/GlobalVariable handling,
more precisely:

LLVM converts llvm::ConstantInt values to GlobalVariable addresses ad-hoc
if I store a constant in a GlobalVariable, and it appears to me, that I can
do nothing about it. Consider the following, shortened code:
0x18: r2 = memread(0x4000)
0x1C: r3 = memread(0x4004)
0x20: r4 = add(r2, r3)
Gets translated to:
  %v0_18 = load i32, i32* inttoptr (i32 16384 to i32*), align 16384
  %v0_20 = load i32, i32* inttoptr (i32 16388 to i32*), align 4
  %v2_24 = add i32 %v0_20, %v0_18
which is absolutely fine, however, in another binary:
0x800004: r2 = memread(0x4000)
0x800008: r3 = memread(0x4004)
0x80000C: r6 = 0x4020
0x800010: r1 = logic_shift_right(r6, 3)
0x800014: r4 = add(r2, r3)
Gets translated to:

@global_var_4020.3 = global i32 0
@global_var_4000.4 = local_unnamed_addr global i32 0
@global_var_4004.5 = local_unnamed_addr global i32 0


%v0_800004 = load i32, i32* @global_var_4400.1, align 4
%v0_800008 = load i32, i32* @global_var_4404.2, align 4
%v0_800010 = lshr (i32 ptrtoint (i32* @global_var_4020.3 to i32), i32 3)
%v2_800014 = add i32 %v0_800008, %v0_800004


The funny thing, as you can see, is that now the address range is nowhere
near 0x4000(just like in the previous example). But LLVM uses global
variables, which happen to have the correct addresses. Even more strange
is, that for a bit shifting operation a global variable address is used as

The IRBuilder code for the memread instruction is the following for both
cases(simplified version):

    int constVal = param1;
    llvm::IRBuilder<>& irb;
    llvm::Value* op1 = llvm::ConstantInt::getSigned(irb.getInt32Ty(),
    auto* pt = llvm::PointerType::get(op1->getType(), 0);
    auto* addr = irb.CreateIntToPtr(op1, pt);
    auto* loaded = irb.CreateLoad(addr);
    irb.CreateStore(loaded, module->getNamedGlobal(regname));

I cannot get LLVM to leave an llvm::Value as ConstantInt in every case, if
I pass it to IRBuilder for processing(for example through a globalvariable
storing instruction).
Do you know how I can tell IRBuilder, or any other part of LLVM, to leave a
supplied or retrieved llvm::Value as const?

Best regards,

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