search

llvm / llvm-project

The LLVM Project is a collection of modular and reusable compiler and toolchain technologies.
http://llvm.org
Other
28.69k stars 11.87k forks source link

Wrong optimization: two parts of ppc_fp128 are swapped #44482

Open llvmbot opened 4 years ago

llvmbot commented 4 years ago
Bugzilla Link 45137
Version trunk
OS Linux
Reporter LLVM Bugzilla Contributor

Extended Description

For a PowerPC target, when two parts of long double (IBM extended double / double-double / ppc_fp128) are accessed directly and their values are visible to the optimizer, they are swapped:

include

include

attribute((optnone)) // imagine it in a separate TU static void opaque(void p) { return p; }

int main() { long double d = 1;

unsigned long l[2];
memcpy(&l, &d, sizeof d);

printf("%016lx %016lx\n", l[0], l[1]);

opaque(&l); // hide it from the optimizer

printf("%016lx %016lx\n", l[0], l[1]);

}

$ clang -target ppc64-linux-gnu -std=c11 -Weverything test.c && qemu-ppc64 /usr/powerpc-linux-gnu/lib64/ld64.so.1 --library-path /usr/powerpc-linux-gnu/lib64 ./a.out 3ff0000000000000 0000000000000000 3ff0000000000000 0000000000000000 $ clang -target ppc64-linux-gnu -std=c11 -Weverything -O3 test.c && qemu-ppc64 /usr/powerpc-linux-gnu/lib64/ld64.so.1 --library-path /usr/powerpc-linux-gnu/lib64 ./a.out 0000000000000000 3ff0000000000000 3ff0000000000000 0000000000000000

clang x86-64 version: clang version 11.0.0 (https://github.com/llvm/llvm-project feb20a159410dfec4055428911c7eb2bc908cf12)

The output line "0000000000000000 3ff0000000000000" is wrong.

llvmbot commented 3 months ago

@llvm/issue-subscribers-backend-powerpc

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [45137](https://llvm.org/bz45137) | | Version | trunk | | OS | Linux | | Reporter | LLVM Bugzilla Contributor | ## Extended Description For a PowerPC target, when two parts of long double (IBM extended double / double-double / ppc_fp128) are accessed directly and their values are visible to the optimizer, they are swapped: ---------------------------------------------------------------------- #include <string.h> #include <stdio.h> __attribute__((optnone)) // imagine it in a separate TU static void *opaque(void *p) { return p; } int main() { long double d = 1; unsigned long l[2]; memcpy(&l, &d, sizeof d); printf("%016lx %016lx\n", l[0], l[1]); opaque(&l); // hide it from the optimizer printf("%016lx %016lx\n", l[0], l[1]); } ---------------------------------------------------------------------- $ clang -target ppc64-linux-gnu -std=c11 -Weverything test.c && qemu-ppc64 /usr/powerpc-linux-gnu/lib64/ld64.so.1 --library-path /usr/powerpc-linux-gnu/lib64 ./a.out 3ff0000000000000 0000000000000000 3ff0000000000000 0000000000000000 $ clang -target ppc64-linux-gnu -std=c11 -Weverything -O3 test.c && qemu-ppc64 /usr/powerpc-linux-gnu/lib64/ld64.so.1 --library-path /usr/powerpc-linux-gnu/lib64 ./a.out 0000000000000000 3ff0000000000000 3ff0000000000000 0000000000000000 ---------------------------------------------------------------------- clang x86-64 version: clang version 11.0.0 (https://github.com/llvm/llvm-project feb20a159410dfec4055428911c7eb2bc908cf12) ---------------------------------------------------------------------- The output line "0000000000000000 3ff0000000000000" is wrong.