Missing Underflow float exceptions in some cases (XuanTie Group explained) #17
Description
Some glibc tests failed because of missing expected underflow float exceptions.
FAIL: math/test-double-fma
FAIL: math/test-float-double-div
FAIL: math/test-float-double-fma
FAIL: math/test-float-double-mul
FAIL: math/test-float-fma
FAIL: math/test-float32-float32x-div
FAIL: math/test-float32-float32x-fma
FAIL: math/test-float32-float32x-mul
FAIL: math/test-float32-float64-div
FAIL: math/test-float32-float64-fma
FAIL: math/test-float32-float64-mul
FAIL: math/test-float32-fma
FAIL: math/test-float32x-float64-div
FAIL: math/test-float32x-float64-fma
FAIL: math/test-float32x-fma
FAIL: math/test-float64-fma
The failing math tests have one thing in common: underflow float exception is missing. Here is the test log from math/test-float-double-div:
glibc-build/math/test-float-double-div
testing float (argument double)
Failure: div_downward_double (-0x4p-128, 0x1.000002p+0): Exception "Underflow" not set
Failure: div_downward_double (0x4p-128, -0x1.000002p+0): Exception "Underflow" not set
Failure: div_upward_double (-0x4p-128, -0x1.000002p+0): Exception "Underflow" not set
Failure: div_upward_double (0x4p-128, 0x1.000002p+0): Exception "Underflow" not set
Test suite completed:
1732 test cases plus 1728 tests for exception flags and
1728 tests for errno executed.
4 errors occurred.
I built the following minimal reproduction out of this:
#include <math.h>
#include <fenv.h>
#include <stdio.h>
int main() {
if (fesetround (FE_DOWNWARD)) {
printf("ERROR: Failed to set rounding mode!\n");
return 1;
}
float ans = fdiv(-0x4p-128, 0x1.000002p+0);
if(fetestexcept (FE_UNDERFLOW)) {
printf("Success: Exception Underflow is set!\n");
} else {
printf("Failure: Exception Underflow is not set!!!\n");
}
}And the following command is used to compile and run it:
gcc fdiv-repro.c -lm -std=c2x -o fdiv-repro && ./fdiv-reproExpected behavior: the program output Success: Exception Underflow is set!.
However, I got Failure: Exception Underflow is not set!!! .
Test results on different machines, OS and architectures:
revyos, lpi4a: Failure: Exception Underflow is not set!!!
Arch, sg2042: Failure: Exception Underflow is not set!!!
Arch, lpi4a: Failure: Exception Underflow is not set!!!
Arch, qemu-user: Success: Exception Underflow is set!
Arch, unmatched: Success: Exception Underflow is set!
Arch, qemu-system: Success: Exception Underflow is set!
Arch, x86_64: Success: Exception Underflow is set!
This might suggest something is wrong in the floating-point implementation of C910 & C920.