Bug #79595 [Opn]: zend_init_fpu() alters FPU precision

Edit report at https://bugs.php.net/bug.php?id=79595&edit=1 ID: 79595 User updated by: v-yitam at microsoft dot com Reported by: v-yitam at microsoft dot com Summary: zend_init_fpu() alters FPU precision Status: Open Type: Bug Package: Math related Operating System: Alpine Linux PHP Version: 7.4.5 Block user comment: N Private report: N New Comment: I wanted to modify my original bug report but it didn't seem possible. Anyway, below is a more accurate example to illustrate the issue. It was copied when running the debugger and stepping through zend_init_fpu() in Zend/zend_float.c: 24 { (gdb) s 28 if (!EG(saved_fpu_cw_ptr)) { (gdb) n 29 EG(saved_fpu_cw_ptr) = (void*)&EG(saved_fpu_cw); (gdb) n 31 XPFPA_STORE_CW(EG(saved_fpu_cw_ptr)); (gdb) n 32 long double n = 1; (gdb) n 33 printf("Before %.17Le\n", n/100000); (gdb) n Before 1.00000000000000000e-05 35 XPFPA_SWITCH_DOUBLE(); (gdb) n 36 printf("After %.17Le\n", n/100000); (gdb) n After 1.00000000000000008e-05 40 } (gdb) Previous Comments: ------------------------------------------------------------------------ [2020-05-13 23:48:13] v-yitam at microsoft dot com Description: ------------ A PHP extension which depends on the FPU precision being the default of the ABI (80 bit) will produce subtly incorrect results with floating point number. In particular, it breaks any musl libc-linked binaries that use floating point strtod(). In Alpine Linux, musl's strtod (https://git.musl-libc.org/cgit/musl/tree/src/internal/floatscan.c) depends on long double, which requires FPU set to 80-bit precision to actually work To illustrate, I wrote two simple test scripts (one cpp and one php), using the example provided in a similar issue (https://github.com/microsoft/WSL/issues/830): The output from the test program: 1.00000000000000000e-05 The output from running php: 1.00000000000000008e-5 Please see below. Test script: --------------- alpine:~$ more test_print.cpp #include <stdio.h> int main() { long double n = 1; printf("%.17Le\n", n / 100000); } alpine:~$ g++ -std=c++11 -static -o test_print test_print.cpp alpine:~$ ./test_print 1.00000000000000000e-05 alpine:~$ more test_print.cpp #include <stdio.h> int main() { long double n = 1; printf("%.17Le\n", n / 100000); } alpine:~$ g++ -std=c++11 -static -o test_print test_print.cpp alpine:~$ ./test_print 1.00000000000000000e-05 alpine:~$ more test_print.php <?php $n = 1; printf("%.17e\n", 1/100000); ?> alpine:~$ php test_print.php 1.00000000000000008e-5 Expected result: ---------------- FPU precision should not be changed Actual result: -------------- Have to save/restore FPU state and force it back to 80-bit ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=79595&edit=1