clang hangs when compile a large c file.

[llvmbot]

Bugzilla Link	25416
Resolution	FIXED
Resolved on	Sep 09, 2016 15:17
Version	unspecified
OS	Linux
Blocks	#24719
Reporter	LLVM Bugzilla Contributor

Extended Description

We have a c file like this
It contains a jmp statement for every 4K nop. There are over
4M instructions in the asm. gcc compile it within one second.
Clang hangs.
The command line is
clang iTLB_benchmark_function.c -c -o iTLB_benchmark_function.o

void iTLB_bechmark_function() {
asm (
"1:jmp 1f\n\t"
"nop\n\t"
"nop\n\t"
.
.
.
"1:nop\n\t"
);
}

[llvmbot]

Is clang -S slow too or just clang -c?

Can you attach the file?

[llvmbot]

test case for it.
The test c file is generated by the script attached.
One may run
./generateBenchmarkFunction.sh 4096 1024 > test.c

after that, one can run
clang -c test.c to reproduce this problem.

[llvmbot]

Is clang -S slow too or just clang -c?

Can you attach the file?

The -S is slow too. gcc actually compile it within 3 seconds.

[llvmbot]

reduced test
This is really a clang bug. In this "small" version of the test:

• clang -cc1 test.c -emit-llvm -o t.ll: takes about 3s
• llvm-as/llvm-dis/llc/llvm-mc are "instantaneous"
• gcc/gas are "instantaneous"

[llvmbot]

Most of time is spend on
static llvm::MDNode *getAsmSrcLocInfo

in the loop
if (!StrVal.empty()) {
const SourceManager &SM = CGF.CGM.getContext().getSourceManager();
const LangOptions &LangOpts = CGF.CGM.getLangOpts();

// Add the location of the start of each subsequent line of the asm to the
// MDNode.

for (unsigned i = 0, e = StrVal.size()-1; i != e; ++i) {

[llvmbot]

This was fixed by the reporter back in r255198. Thanks for the patch!