<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
<p><font face="Hack Nerd Font Mono">Right.</font></p>
<p><font face="Hack Nerd Font Mono"><br>
</font></p>
<p><font face="Hack Nerd Font Mono">Maybe I can put this a bit
differently:<br>
</font></p>
<p><font face="Hack Nerd Font Mono">A `llvm::GlobalVariable` is
*not* the source variable but a pointer to the storage location
of the source variable.</font></p>
<p>This is the same for an `llvm::AllocaInst` that represents a
local variable.</p>
<p> Using a source variable means reading the `llvm::GlobalVariable`
(or `llvm::AllocaInst`) that models the memory allocated for the
source variable.</p>
<p> Changing the variable corresponds to a write.<br>
</p>
<p><br>
</p>
<p>Hope this helps :)<br>
</p>
<p><br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 6/3/20 1:56 PM, David Blaikie via
llvm-dev wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAENS6EsorG+4UsA8v-96NqhqgQhC+G1ZCi8aYvRD+579b_yRLw@mail.gmail.com">
<pre class="moz-quote-pre" wrap="">I'm with Eli on this "GlobalVariable" is not the actual global
variable in memory - it's the IR representation of the global variable
before it is compiled to machine code & allocated in the running
process - it always has the same value/isn't affected by the execution
of loads and stores at runtime.
On Wed, Jun 3, 2020 at 2:53 AM Emma Luciano via llvm-dev
<a class="moz-txt-link-rfc2396E" href="mailto:llvm-dev@lists.llvm.org"><llvm-dev@lists.llvm.org></a> wrote:
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">
I don't think it's the same problem as you described. By printing I meant calling printf function and passing my global variable as one of the arguments.
My code:
Instruction* InstructionVisitor::incrementGlobalKey(Instruction* I) {
IRBuilder<> Builder(I->getContext());
Builder.SetInsertPoint(I->getNextNode());
GlobalVariable* key = I->getModule()->getNamedGlobal("globalKey");
if (key) {
LoadInst* load = Builder.CreateLoad(key);
Value* inc = Builder.CreateAdd(load, Builder.getInt64(1));
StoreInst* store = Builder.CreateStore(inc, key);
return store;
}
return I;
}
Instruction* InstructionVisitor::print(Instruction* I, const char* text, Value* arg1, Value* arg2, Value* arg3, Value* arg4) {
Function* printfFn = I->getModule()->getFunction("printf");
if (printfFn) {
IRBuilder<> Builder(I->getContext());
Builder.SetInsertPoint(I->getNextNode());
Value* convertedText = Builder.CreateGlobalStringPtr(text);
std::vector <Value *> params;
params.push_back(convertedText);
if (arg1)
params.push_back(arg1);
if (arg2)
params.push_back(arg2);
if (arg3)
params.push_back(arg3);
if (arg4)
params.push_back(arg4);
return Builder.CreateCall(printfFn, params);
}
return I;
}
void InstructionVisitor::visitCallInst(CallInst &CI) {
if (isAllocationFn(&CI, &TLI)) {
Value* allocatedAddress = &CI;
Instruction* I = &CI;
Value* allocatedSize = I->getOperand(0);
Instruction* next = incrementGlobalKey(I);
GlobalVariable* key = I->getModule()->getNamedGlobal("globalKey");
const char* message = "Allocated address: 0x%p, size: %d, key: %lld\n";
print(next, message, allocatedAddress, allocatedSize, key->getOperand(0));
}
}
Code that I'm executing (source code) is:
int main()
{
char* buffer;
int size = 101;
buffer = new char[size];
printf("CHECK address: 0x%p, size: %d\n", buffer, size);
TestClass* test = new TestClass(1, 2);
printf("(CHECK address: 0x%p, size: %llu\n", test, sizeof(TestClass));
delete buffer;
delete test;
}
And program output after instrumentation:
Allocated address: 0x000001CD326C1B20, size: 101, key: 0
CHECK address: 0x000001CD326C1B20, size: 101
Allocated address: 0x000001CD326C1B90, size: 8, key: 0
CHECK address: 0x000001CD326C1B90, size: 8
Regards,
Emma Luciano
śr., 3 cze 2020 o 11:21 Eli Friedman <a class="moz-txt-link-rfc2396E" href="mailto:efriedma@quicinc.com"><efriedma@quicinc.com></a> napisał(a):
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">
Suppose I have the following program:
$ cat test.c
int a = 0;
int main() { a = 1; };
Then I compile and run it:
$ clang test.c
$ ./a.out
Then I print the contents of test.c again:
$ cat test.c
int a = 0;
int main() { a = 1; }
Why does it say “int a = 0;”? The program set it to 1, no?
As far as I can tell, this is equivalent to your question… except it’s less obvious because the “source code” is an LLVM IR module in memory.
Maybe you’re looking for ExecutionSession::lookup? The same way the tutorial computes the address of “main”, you can compute the runtime address of any other externally visible symbol.
See also <a class="moz-txt-link-freetext" href="https://llvm.org/docs/ORCv2.html">https://llvm.org/docs/ORCv2.html</a> , <a class="moz-txt-link-freetext" href="https://www.youtube.com/watch?v=hILdR8XRvdQ">https://www.youtube.com/watch?v=hILdR8XRvdQ</a> .
-Eli
From: llvm-dev <a class="moz-txt-link-rfc2396E" href="mailto:llvm-dev-bounces@lists.llvm.org"><llvm-dev-bounces@lists.llvm.org></a> On Behalf Of Emma Luciano via llvm-dev
Sent: Wednesday, June 3, 2020 1:35 AM
To: <a class="moz-txt-link-abbreviated" href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>
Subject: [EXT] [llvm-dev] Fwd: I cannot change value of global variable in LLVM IR using IRBuilder
Hi Everyone,
I'm quite new to LLVM and I want to update value of global variable in LLVM IR. I created new global variable in ModulePass:
bool runOnModule(llvm::Module &M) {
IRBuilder<> Builder(M.getContext());
Instruction *I = &*inst_begin(M.getFunction("main"));
Builder.SetInsertPoint(I);
M.getOrInsertGlobal("globalKey", Builder.getInt64Ty());
GlobalVariable* gVar = M.getNamedGlobal("globalKey");
gVar->setLinkage(GlobalValue::InternalLinkage);
gVar->setAlignment(Align(8));
gVar->setInitializer(Builder.getInt64(0));
gVar->setConstant(false);
for (Function &F : M.functions()) {
InstructionVisitor visitor(DL, getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F));
for (Instruction &I : instructions(F)) {
visitor.visit(I);
}
}
return true;
}
Later in InstructionVisitor I try to update it like that:
IRBuilder<> Builder(I->getContext());
Builder.SetInsertPoint(I->getNextNode());
GlobalVariable* key = I->getModule()->getNamedGlobal("globalKey");
if (key) {
LoadInst* load = Builder.CreateLoad(key);
Value* inc = Builder.CreateAdd(load, Builder.getInt64(1));
StoreInst* store = Builder.CreateStore(inc, key);
}
I print that global variable during execution of instrumented code. I access it's value by key->getOperand(0), but it's unchanged. I'm using ORC JIT based on this tutorial: <a class="moz-txt-link-freetext" href="https://llvm.org/docs/tutorial/BuildingAJIT2.html">https://llvm.org/docs/tutorial/BuildingAJIT2.html</a> and I run ModulePass from optimizeModule function from this tutorial.
In IR it looks like that:
%2 = load i64, i64* @globalKey
%3 = add i64 %2, 1
store i64 %3, i64* @globalKey
I tried updating value of global variable, which was present in source code that I'm instrumenting. It didn't work either.
I created corresponding stackoverflow topic: <a class="moz-txt-link-freetext" href="https://stackoverflow.com/questions/62142574/how-can-i-update-global-variable-value-in-llvm-ir-using-irbuilder?fbclid=IwAR2X7uGhUjm_SN2UcxWYqaRQW2-cs6iPbMx2_tbKubeyYUkZ_pS6rkFfY9I">https://stackoverflow.com/questions/62142574/how-can-i-update-global-variable-value-in-llvm-ir-using-irbuilder?fbclid=IwAR2X7uGhUjm_SN2UcxWYqaRQW2-cs6iPbMx2_tbKubeyYUkZ_pS6rkFfY9I</a>
I will be really grateful for help.
Regards,
Emma Luciano
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">
_______________________________________________
LLVM Developers mailing list
<a class="moz-txt-link-abbreviated" href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>
<a class="moz-txt-link-freetext" href="https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev">https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a>
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">_______________________________________________
LLVM Developers mailing list
<a class="moz-txt-link-abbreviated" href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>
<a class="moz-txt-link-freetext" href="https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev">https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a>
</pre>
</blockquote>
</body>
</html>