<table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Issue</th>
<td>
<a href=http://email.email.llvm.org/c/eJzVWVlz2zgS_jXyC0oqHjof_CDLVkpbdsYbZzJ5c0EkJGJDEhoCtOz59dvd4C0qkSe7lV2Xwkg4ur8-2P0R3Krw7XrDIv4iWKqMDETItEoEk2mgUi21EWkghWZbYY5CpMxEgq0G3hr-3cCHLR83A2-mB96K8TSk6VAFeSJSw41UKU7IlGViz7NQM6PYMeKG6UjlcQh6DwcQeozgooUxMt0zHst9ivuZSlkgMsNhv0y1yfIAJZKQv0SmRmyTssHk5lTn5HbgzSNjDnrgLwEsfOL4JRmpbA9fYS3gXd_zdP9J7EaRSeKB52ujMjFs6Bl4C7ZTGZlEk6w1CWaBGaBQkBLnduCUV_-OLdkLj3PB1I45DITwlKlEGgPuJfsYz_YEmCWCo0noFNSkDiIjI8A3mkaWN5uGT0pEBgQIM2rq_SOSQcRkcogxYCQR4kouTZl4heEAIA-mjgP_mNQIUYYMXAUrY8G1wUhtPoHhaJ6BEO0jG5s3lTOTvaHnQwUzsFmezwV_ib4REuYzFmRcR4ISBKDv5QtgOwqZhZATOo-NhjDCIsFNmXoPXx5RDCpD1BD-joOfTjIFlrpoyFFl3zTirzZMneJjf3pjNgLECQqlkVDsOKCoZSH6sZ3bK_o18DxWZsB0zFzfn1H4p-OBh7MTh7KfwgpDnt2cimNbqPu3hbqV0FMrhsHh8H9pSScw5wPrYGBDJTTcdDPDMMT_oxH23PFsPPen4_k56dxAFQjOKYES-OdzoE2trCHxl6ZApYYkrHOdwRVaRJyHoiiu8N9TfjioDOCvlyWAUQRbZjPcR4sgav6SZptfvXkuUzMdPxsbWbbUGgo_VkEI7BeqpFhVMRWm8EEr7GiSQ9WCxgX9iTkI15uxHZexCIvSuNwCIkFVLkCjwzw5iBDVnE9D-EnpA1Uy18JW4ZWtbrbLFQvII61FdSEcsSeVZ4HAOrsVsTq2SvXniIaxPENR1EIkVO6g2yZsl6mEtZtXJl6kOOpRo4ndzmbjyXjE2pKO0DPQGweorXj7QJG-v__ywFyK6jY3zFbuF5Fp9K7rQpM9Qmcedezv3lJ5yqEyD7kduZdp_gotLMACPxm505E3xF_uEKuw77Knh0f2-Onu7uHxMyDMUbnnsH9AH_IcSEB3Cqa5c7hAjwWHOfDHXufTZ8jIDx9_x-aMGroJf-BBAiKG_9Q2mWgiVgGH_r0OYujnkM8jZ-QMi5xn8LdiOJHzvYDESGT8hg5OgdaE1EvRPW1BMfCSQoxbiwFA38BYCg6Gmtx6yNS_RGC6-1-SUsC0gUPFsSDugKQgUWEe84yyKRO55ttYsEAlB0hdO2qUioMIuY8RQZSqWO2hrZ-qGsZyq3v0EcAsT42EwMGajGfV_v6s91ehALIVQ97dVUM6TxKevcEQ3QHQkTkQDAEzoL-e7E2foAydX5QKhgJNuAXTsDDc9c9n0AK-Mx1K1ZztWUNuCZCYwB1_slYi6cLb25u_KImlgA1mN-WKBfpthQnyaoAdssC8gshb1hheEVmB7VSsblo47P4HjK7A7RRnUUmwE1bAH8CPPsJdtUkLuSixlIIfz7O7qaytmlMAqv55DsTnt4OFALY-7_I0eDYwUkFZwwhmIy7raMbpDdZjnGvCQ71EDpurP_5-f98Zcjq_dzzW4seIqaS3IFdol2FYAu6ALX206vqOQtz1XNsZP4Z0w7UMbuB--4a4oKQCCa4w4bNLWK84F0jyWj-KU9ikotHsvwMtl3EoMsS1tV_rRKUEKxb0BNBvp_uj0hI9W2xYmrsUbot5IRWLd4HqR3FL86R5s2gDWfRTuYXMAJ5C9yl08FhBfccLLZtb5tLd4L4jy3iMdbQCTNYvaayDuXbEz1jiXRTVCp3laC1wTzh0ETYIRGfEGntG_-IANdfsqKR5vSQSiGE-mKyKO4ogfRCmonmwU1twXcnQwiAvvSWU9cqYR9RGhn5WT1Tum_vPwCr562RFGCa6-FaDAhVn3frUC3aFnPGsxhN-_7ed8AuMn932t3rq2yd9_jICULPbd9IA2nfaqT2veHDAM4d1XUihZVe3ytnlVF94avRFq8vmcdHizaeiEl60usEMLlpvOcBFS7GyNBf2cB2pIPKCJ2foUiIS1YxMD9XqhC3X-ASOdF8D3xbFY1vrnmowqC574jk8xpScCbiafcpL-DfxnKfyzxwzpuEw1N3lUVYGtPXvybBOLLc3mj3WmtMuRyKbXfKM2CrymNggvJB_KrEpF-Z7OFaLX5Gqveg2w9Zc0UtaCjs9BBwN9_W7udUJyhZAC6AmtefZUltxe__dqxFZymN8ToJnrYv5mApHhV-qR_IzVtT1AW3ZbsmIerBrRuFDTIiaVMHPBvdqpUiRHUOI-RMGAw8gHpWksr7dftevJe0pS9KG2kUd8QoKVE6Sh4ctK1YxmHOiGwQFe5HlJoDsJ9Ooy84LatIeBJN_nFM1R0F8RE964J0lI-_TWN2ytmxRx2vErz5eyvjx2a55Loojy7SiLpudNGY89IeQU7OFJbRwxUyWi14Q7yVKlYJ9myj0cYTCvEDlhlH9odzttH6vnLKcolzV5gM9vitg6DaM8vjNOZeHJXMpcuQ_5bofUatf7Lb_Anl6gD6zkxcypqRcXDbtgefCri1W0LWOmntW6_vlhyc8RMMwDckdt4E7Z8O9z4a_FchXX7_2rrPMzJ2drKag05Fai7zRFfv38-q3j-vNhyI3xnhzRSKO8QgrwdOr4QuzpzAq3cl9Vdvt1gJKZ6v93hCO3GI4DHYx3-uOhMqcC2W8vvZIub99lxA8Z2s0qfoK1QxTDRsLa5_H22ti32DR3CioMteqWa1Qup1iQ2WlFFMEjxhWA1HfYGFJB13z0N4qqHGV2r9-rdUfDiUA3MWqFb0w-of7gQSx4M13eIss6fXW9--2q_DaDxf-gl9B74lUdh0qLbUKr_Isvm6fl--lifLtCJKxfjlB6Vge2HprqXWObyXXE-jC7lV0PXeD6cIJxv5k7IyDYLpbBH4w3k13O4-Pt9PFVcy3ItbXg8nNYHJ79fMq5TWehTuu57sL3x87o-l0CpVYLNzFLpiJ0BuMHQEeiqtj_6vsmkRuc0jlsRNLbep3Aldc00GJKBEaaWL6QUfck1u2qV_pm8Yrg-a798Yb7s5LuCtCf03Q_w2r5x95>53371</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>
[LLVM] Inconsistent behavior when setting instruction alignment to 0
</td>
</tr>
<tr>
<th>Labels</th>
<td>
</td>
</tr>
<tr>
<th>Assignees</th>
<td>
</td>
</tr>
<tr>
<th>Reporter</th>
<td>
dosisod
</td>
</tr>
</table>
<pre>
I have noticed some inconsistencies between the C/C++ API's, and the documentation, in regards to what should happen when setting alignment on certain instructions to zero. In [the documentation](https://llvm.org/docs/LangRef.html#store-instruction) for the store instruction, it states:
> A value of 0 or an omitted align argument means that the operation has the ABI alignment for the target.
Which implies that having an explicit `0` is valid (at least in IR), though when you try to do this in the C/C++ API's: it either crashes, or gives weird results. I created some MVP's to explain:
Setting alignment to 1 (works):
```
$ ./main
default alignment: 4
got: " store i64 1337, i64* %0, align 4"
new alignment: 1
got: " store i64 1337, i64* %0, align 1"
$ ./main-cpp
default alignment: 4
got: " store i64 1337, i64* %0, align 4"
new alignment: 1
got: " store i64 1337, i64* %0, align 1"
```
Setting alignment to 0 (doesn't work):
```
$ ./main
default alignment: 4
got: " store i64 1337, i64* %0, align 4"
new alignment: 2147483648
got: " store atomic i64 1337, i64* %0 seq_cst, align 2147483648"
$ ./main-cpp
default alignment: 4
got: " store i64 1337, i64* %0, align 4"
main-cpp: /usr/include/llvm/Support/Alignment.h:77: llvm::Align::Align(uint64_t): Assertion `Value > 0 && "Value must not be 0"' failed.
Aborted (core dumped)
```
`main` uses the C API, and `main-cpp` uses the C++ API. Source is below.
This behavior seems to stem from https://reviews.llvm.org/D77454. This behavior was not present in LLVM 10, but is in version 11 onward.
```
$ uname -a
Linux archer 5.16.2-arch1-1 #1 SMP PREEMPT Thu, 20 Jan 2022 16:18:29 +0000 x86_64 GNU/Linux
$ pacman -Qs llvm
local/clang 13.0.0-4
C language family frontend for LLVM
local/lld 13.0.0-1
Linker from the LLVM project
local/llvm 13.0.0-6
Collection of modular and reusable compiler and toolchain technologies
local/llvm-libs 13.0.0-6
LLVM runtime libraries
```
<details>
<summary>C API example</summary>
```c
#include <stdbool.h>
#include <string.h>
#include <stdio.h>
#include <llvm-c/Core.h>
int main(void) {
LLVMContextRef ctx = LLVMContextCreate();
LLVMModuleRef module = LLVMModuleCreateWithNameInContext(
"module",
ctx
);
LLVMTypeRef main_func_type = LLVMFunctionType(
LLVMInt64TypeInContext(ctx),
NULL,
0,
false
);
LLVMValueRef main_func = LLVMAddFunction(
module,
"main",
main_func_type
);
LLVMBasicBlockRef entry = LLVMAppendBasicBlockInContext(
ctx,
main_func,
"entry"
);
LLVMBuilderRef builder = LLVMCreateBuilderInContext(ctx);
LLVMPositionBuilderAtEnd(builder, entry);
LLVMValueRef num = LLVMConstInt(
LLVMInt64TypeInContext(ctx),
(unsigned long long)(1337),
1
);
LLVMValueRef alloca = LLVMBuildAlloca(
builder,
LLVMInt64TypeInContext(ctx),
""
);
LLVMValueRef store = LLVMBuildStore(
builder,
num,
alloca
);
printf("default alignment: %u\n", LLVMGetAlignment(store));
char *str = LLVMPrintValueToString(store);
printf("got: \"%s\"\n", str);
LLVMSetAlignment(store, 0);
printf("new alignment: %u\n", LLVMGetAlignment(store));
str = LLVMPrintValueToString(store);
printf("got: \"%s\"\n", str);
}
```
</details>
<details>
<summary>C++ API example</summary>
```c++
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include <iostream>
#include <memory>
#include <string>
using namespace llvm;
int main() {
auto ctx = std::make_unique<LLVMContext>();
auto mod = std::make_unique<Module>("main", *ctx);
auto builder = std::make_unique<IRBuilder<>>(*ctx);
auto *main_func_type = FunctionType::get(
Type::getInt64Ty(*ctx),
{},
false
);
auto *main_func = Function::Create(
main_func_type,
Function::ExternalLinkage,
"main",
mod.get()
);
BasicBlock *bb = BasicBlock::Create(*ctx, "entry", main_func);
builder->SetInsertPoint(bb);
auto *num = ConstantInt::get(*ctx, APInt(64, 1337));
auto alloca = new AllocaInst(
Type::getInt64Ty(*ctx),
0,
"",
bb
);
auto store = new StoreInst(
num,
alloca,
bb
);
std::string str;
llvm::raw_string_ostream rso(str);
store->print(rso, true);
printf("default alignment: %u\n", store->getAlignment());
std::cout << "got: \"" << str << "\"\n";
store->setAlignment(Align(0));
str = "";
store->print(rso, true);
printf("new alignment: %u\n", store->getAlignment());
std::cout << "got: \"" << str << "\"\n";
}
```
</details>
<details>
<summary>Makefile</summary>
```makefile
#!/bin/sh
CFLAGS += -std=c18 -g3 -O0
CXXFLAGS += -std=c++17 -g3 -O0
CXX = clang++
LLVM_CONFIG = $(shell command -v llvm-config)
LLVM_CFLAGS = $(shell $(LLVM_CONFIG) --cflags)
LLVM_CXXFLAGS = $(shell $(LLVM_CONFIG) --cxxflags)
LLVM_LDFLAGS = $(shell $(LLVM_CONFIG) --libs)
all: main main-cpp
main: main.c
$(CC) main.c -o main $(CFLAGS) $(LLVM_CFLAGS) $(LLVM_LDFLAGS)
main-cpp: main.cpp
$(CXX) main.cpp -o main-cpp $(CXXFLAGS) $(LLVM_CXXFLAGS) $(LLVM_LDFLAGS)
clean:
rm main main-cpp
```
</details>
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJzVWVlz2zgS_jXyC0oqHjof_CDLVkpbdsYbZzJ5c0EkJGJDEhoCtOz59dvd4C0qkSe7lV2Xwkg4ur8-2P0R3Krw7XrDIv4iWKqMDETItEoEk2mgUi21EWkghWZbYY5CpMxEgq0G3hr-3cCHLR83A2-mB96K8TSk6VAFeSJSw41UKU7IlGViz7NQM6PYMeKG6UjlcQh6DwcQeozgooUxMt0zHst9ivuZSlkgMsNhv0y1yfIAJZKQv0SmRmyTssHk5lTn5HbgzSNjDnrgLwEsfOL4JRmpbA9fYS3gXd_zdP9J7EaRSeKB52ujMjFs6Bl4C7ZTGZlEk6w1CWaBGaBQkBLnduCUV_-OLdkLj3PB1I45DITwlKlEGgPuJfsYz_YEmCWCo0noFNSkDiIjI8A3mkaWN5uGT0pEBgQIM2rq_SOSQcRkcogxYCQR4kouTZl4heEAIA-mjgP_mNQIUYYMXAUrY8G1wUhtPoHhaJ6BEO0jG5s3lTOTvaHnQwUzsFmezwV_ib4REuYzFmRcR4ISBKDv5QtgOwqZhZATOo-NhjDCIsFNmXoPXx5RDCpD1BD-joOfTjIFlrpoyFFl3zTirzZMneJjf3pjNgLECQqlkVDsOKCoZSH6sZ3bK_o18DxWZsB0zFzfn1H4p-OBh7MTh7KfwgpDnt2cimNbqPu3hbqV0FMrhsHh8H9pSScw5wPrYGBDJTTcdDPDMMT_oxH23PFsPPen4_k56dxAFQjOKYES-OdzoE2trCHxl6ZApYYkrHOdwRVaRJyHoiiu8N9TfjioDOCvlyWAUQRbZjPcR4sgav6SZptfvXkuUzMdPxsbWbbUGgo_VkEI7BeqpFhVMRWm8EEr7GiSQ9WCxgX9iTkI15uxHZexCIvSuNwCIkFVLkCjwzw5iBDVnE9D-EnpA1Uy18JW4ZWtbrbLFQvII61FdSEcsSeVZ4HAOrsVsTq2SvXniIaxPENR1EIkVO6g2yZsl6mEtZtXJl6kOOpRo4ndzmbjyXjE2pKO0DPQGweorXj7QJG-v__ywFyK6jY3zFbuF5Fp9K7rQpM9Qmcedezv3lJ5yqEyD7kduZdp_gotLMACPxm505E3xF_uEKuw77Knh0f2-Onu7uHxMyDMUbnnsH9AH_IcSEB3Cqa5c7hAjwWHOfDHXufTZ8jIDx9_x-aMGroJf-BBAiKG_9Q2mWgiVgGH_r0OYujnkM8jZ-QMi5xn8LdiOJHzvYDESGT8hg5OgdaE1EvRPW1BMfCSQoxbiwFA38BYCg6Gmtx6yNS_RGC6-1-SUsC0gUPFsSDugKQgUWEe84yyKRO55ttYsEAlB0hdO2qUioMIuY8RQZSqWO2hrZ-qGsZyq3v0EcAsT42EwMGajGfV_v6s91ehALIVQ97dVUM6TxKevcEQ3QHQkTkQDAEzoL-e7E2foAydX5QKhgJNuAXTsDDc9c9n0AK-Mx1K1ZztWUNuCZCYwB1_slYi6cLb25u_KImlgA1mN-WKBfpthQnyaoAdssC8gshb1hheEVmB7VSsblo47P4HjK7A7RRnUUmwE1bAH8CPPsJdtUkLuSixlIIfz7O7qaytmlMAqv55DsTnt4OFALY-7_I0eDYwUkFZwwhmIy7raMbpDdZjnGvCQ71EDpurP_5-f98Zcjq_dzzW4seIqaS3IFdol2FYAu6ALX206vqOQtz1XNsZP4Z0w7UMbuB--4a4oKQCCa4w4bNLWK84F0jyWj-KU9ikotHsvwMtl3EoMsS1tV_rRKUEKxb0BNBvp_uj0hI9W2xYmrsUbot5IRWLd4HqR3FL86R5s2gDWfRTuYXMAJ5C9yl08FhBfccLLZtb5tLd4L4jy3iMdbQCTNYvaayDuXbEz1jiXRTVCp3laC1wTzh0ETYIRGfEGntG_-IANdfsqKR5vSQSiGE-mKyKO4ogfRCmonmwU1twXcnQwiAvvSWU9cqYR9RGhn5WT1Tum_vPwCr562RFGCa6-FaDAhVn3frUC3aFnPGsxhN-_7ed8AuMn932t3rq2yd9_jICULPbd9IA2nfaqT2veHDAM4d1XUihZVe3ytnlVF94avRFq8vmcdHizaeiEl60usEMLlpvOcBFS7GyNBf2cB2pIPKCJ2foUiIS1YxMD9XqhC3X-ASOdF8D3xbFY1vrnmowqC574jk8xpScCbiafcpL-DfxnKfyzxwzpuEw1N3lUVYGtPXvybBOLLc3mj3WmtMuRyKbXfKM2CrymNggvJB_KrEpF-Z7OFaLX5Gqveg2w9Zc0UtaCjs9BBwN9_W7udUJyhZAC6AmtefZUltxe__dqxFZymN8ToJnrYv5mApHhV-qR_IzVtT1AW3ZbsmIerBrRuFDTIiaVMHPBvdqpUiRHUOI-RMGAw8gHpWksr7dftevJe0pS9KG2kUd8QoKVE6Sh4ctK1YxmHOiGwQFe5HlJoDsJ9Ooy84LatIeBJN_nFM1R0F8RE964J0lI-_TWN2ytmxRx2vErz5eyvjx2a55Loojy7SiLpudNGY89IeQU7OFJbRwxUyWi14Q7yVKlYJ9myj0cYTCvEDlhlH9odzttH6vnLKcolzV5gM9vitg6DaM8vjNOZeHJXMpcuQ_5bofUatf7Lb_Anl6gD6zkxcypqRcXDbtgefCri1W0LWOmntW6_vlhyc8RMMwDckdt4E7Z8O9z4a_FchXX7_2rrPMzJ2drKag05Fai7zRFfv38-q3j-vNhyI3xnhzRSKO8QgrwdOr4QuzpzAq3cl9Vdvt1gJKZ6v93hCO3GI4DHYx3-uOhMqcC2W8vvZIub99lxA8Z2s0qfoK1QxTDRsLa5_H22ti32DR3CioMteqWa1Qup1iQ2WlFFMEjxhWA1HfYGFJB13z0N4qqHGV2r9-rdUfDiUA3MWqFb0w-of7gQSx4M13eIss6fXW9--2q_DaDxf-gl8ZaWJxPZjc0OHt5JZt6pfVpnEY3nyr3Hh323m9dJVn8XX7vH0vTZRvR5DM9csNSufywNdbS61zfKu5nkAXd6-ia9cJg23oi5B7s9lOjHeBt3Dn40BM_cV06jtXMd-KWCNsgHwlr_Ek3HE93134_tgZTadTqMNi4S52wUyE3mDsCPBPXB36X2XXhGGbQyKPnRjMrd8IXHFNxySilA_9OVLZdajALSq8IrjXhPXfWL0KMQ">