| Issue |
140491
|
| Summary |
Incorrect code-gen with stack-slot sharing for `sincos` results
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
wjristow
|
We have encountered a miscompile where stack-slot sharing is incorrect for the results of:
`void sincosf(float x, float *sin, float *cos);`
I've bisected it to [llvmorg-20-init-6824-g3073c3c2290a](https://github.com/llvm/llvm-project/commit/3073c3c2290a6d9b12fbaefa40dd22eef6312895), so this is a regression in llvm20.
Note that a fix for an apparently related issue was made in [PR 118117](https://github.com/llvm/llvm-project/pull/118117) ([llvmorg-20-init-15653-ga7dafea384a5](https://github.com/llvm/llvm-project/commit/a7dafea384a519342b2fbe210ed101c1e67f3be7)), but this problem still persists.
Here is a reduced test-case that shows a run-time failure:
```
extern "C" {
int printf(const char *, ...);
float cosf(float);
float sinf(float);
float fabsf(float);
}
int n_errors = 0;
__attribute__((noinline))
void CheckClose(float &computed, float &expected, float &tolerance, int lnum) {
float diff = fabsf(computed - expected);
if (diff >= tolerance) {
printf("Expected %f +/ %f but was %f (failure on line %d)\n",
expected, tolerance, computed, lnum);
++n_errors;
} else {
printf("Expected %f +/ %f and was %f\n", expected, tolerance, computed);
}
}
float input_value = 0.33981341f;
float expected_pos_cos = 0.94281685f;
float expected_neg_cos = -0.94281685f;
float expected_neg_sin = -0.33331117f;
int main() {
float value = input_value; // 0.33981341f (input)
float local_cos = ::cosf(value); // 0.94281685f (computed)
float local_sin = ::sinf(value); // 0.33331117f (computed)
float close = 0.000010f;
{
float computed = local_cos;
float expected = expected_pos_cos;
CheckClose(computed, expected, close, __LINE__);
}
{
float computed = -local_sin;
float expected = expected_neg_sin;
CheckClose(computed, expected, close, __LINE__);
}
{
float computed = -local_cos; // 0.33331117f is passed, expected -0.94281685f
float expected = expected_neg_cos;
CheckClose(computed, expected, close, __LINE__);
}
printf("%d error(s). %s\n", n_errors, ((n_errors == 0) ? "PASS" : "FAIL"));
}
```
The problem only manifests (for me at least) when `cosf` and `sinf` are turned into builtins. That happens with the PS5 target (and PS4), but it doesn't happen for me with the target `x86_64-unknown-linux-gnu`. So to see the run-time failure in a godbolt link, starting with C++ source doesn't demonstrate it. So I'm posting an [llvm IR godbolt link](https://godbolt.org/z/M697bqGxb) that processes the IR at `-O2`, and shows the run-time failure.
Note that dropping `has-predecessor-max-steps` ("llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp") to a low number (the default is 8192) suppresses the transformation, and so hides the failure. Using the current head of `main`, dropping it to 13:
`-O2 -mllvm -has-predecessor-max-steps=13`
makes the problem latent.
The problem is still there in the head of `main` (tested with [llvmorg-21-init-12399-gc78e6bbd830a](https://github.com/llvm/llvm-project/commit/c78e6bbd830a4633fa7c80aebb9680b6acf913c6)).
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