[Bug c/84377] New: gcc-7.3.0 miscompiles truncf128@@GLIBC_2.26 in libm.so

["dilfridge at gentoo dot org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84377

            Bug ID: 84377
           Summary: gcc-7.3.0 miscompiles truncf128@@GLIBC_2.26 in libm.so
           Product: gcc
           Version: 7.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dilfridge at gentoo dot org
  Target Milestone: ---

Created attachment 43411
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43411&action=edit
preprocessed source

I stumbled on this when trying to track down failures in the glibc-2.26
testsuite on Gentoo.
See also for the initial report 
https://sourceware.org/bugzilla/show_bug.cgi?id=22826

* Problem occurs only with gcc-7.3.0, not gcc-6.4.0 with identical settings
* Problem occurs only when the test program loads libm.so compiled by said
compiler; which compiler is used to compile the test binary doesn't matter.
* Problem occurs (at least) when building a -m32 library in an x86-64 system; a
pure 32 bit install seems to be fine (but it has different CFLAGS etc).
* The test reports that truncf returns completely wrong data. Example:

testing _Float128 (without inline functions)
Failure: Test: trunc (1.625)
Result:
 is:          1.62500000000000000000000000000000000e+00  
0x1.a000000000000000000000000000p+0
 should be:   1.00000000000000000000000000000000000e+00  
0x1.0000000000000000000000000000p+0
 difference:  6.25000000000000000000000000000000000e-01  
0x1.4000000000000000000000000000p-1
 ulp       :  3245185536584267267831560205762560.0000
 max.ulp   :  0.0000

Compiler call: 

x86_64-pc-linux-gnu-gcc -m32 -Wl,-O1 -Wl,--as-needed --save-temps
../sysdeps/ieee754/float128/s_truncf128.c -c -std=gnu11 -fgnu89-inline  -O2
-Wall -Wundef -Wwrite-strings -fmerge-all-constants -fno-strict-aliasing
-frounding-math -fstack-protector-all -ggdb -march=native -Wstrict-prototypes
-Wold-style-definition -mpreferred-stack-boundary=4  -fPIC -Wa,-mtune=i686     
 -U_FORTIFY_SOURCE  -D__NO_MATH_INLINES -D__LIBC_INTERNAL_MATH_INLINES
-I../include
-I/var/tmp/portage/sys-libs/glibc-2.26-r6/work/build-x86-x86_64-pc-linux-gnu-nptl/math

-I/var/tmp/portage/sys-libs/glibc-2.26-r6/work/build-x86-x86_64-pc-linux-gnu-nptl
 -I../sysdeps/unix/sysv/linux/i386/i686  -I../sysdeps/i386/i686/nptl 
-I../sysdeps/unix/sysv/linux/i386  -I../sysdeps/unix/sysv/linux/x86 
-I../sysdeps/x86/nptl  -I../sysdeps/i386/nptl 
-I../sysdeps/unix/sysv/linux/include -I../sysdeps/unix/sysv/linux 
-I../sysdeps/nptl  -I../sysdeps/pthread  -I../sysdeps/gnu 
-I../sysdeps/unix/inet  -I../sysdeps/unix/sysv  -I../sysdeps/unix/i386 
-I../sysdeps/unix  -I../sysdeps/posix  -I../sysdeps/i386/i686/fpu/multiarch 
-I../sysdeps/i386/i686/fpu  -I../sysdeps/i386/i686/multiarch 
-I../sysdeps/i386/i686  -I../sysdeps/i386/fpu  -I../sysdeps/x86/fpu/include
-I../sysdeps/x86/fpu  -I../sysdeps/i386  -I../sysdeps/x86 
-I../sysdeps/wordsize-32  -I../sysdeps/ieee754/float128 
-I../sysdeps/ieee754/ldbl-96/include -I../sysdeps/ieee754/ldbl-96 
-I../sysdeps/ieee754/dbl-64  -I../sysdeps/ieee754/flt-32  -I../sysdeps/ieee754 
-I../sysdeps/generic  -I.. -I../libio -I. -nostdinc -isystem
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/include -isystem
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/include-fixed -isystem /usr/include 
-D_LIBC_REENTRANT -include
/var/tmp/portage/sys-libs/glibc-2.26-r6/work/build-x86-x86_64-pc-linux-gnu-nptl/libc-modules.h
-DMODULE_NAME=libm -include ../include/libc-symbols.h  -DPIC -DSHARED    
-DTOP_NAMESPACE=glibc -I../soft-fp -o
/var/tmp/portage/sys-libs/glibc-2.26-r6/work/build-x86-x86_64-pc-linux-gnu-nptl/math/s_truncf128.os
-MD -MP -MF
/var/tmp/portage/sys-libs/glibc-2.26-r6/work/build-x86-x86_64-pc-linux-gnu-nptl/math/s_truncf128.os.dt
-MT
/var/tmp/portage/sys-libs/glibc-2.26-r6/work/build-x86-x86_64-pc-linux-gnu-nptl/math/s_truncf128.os

Preprocessed source will be attached. 

Compiler version: 
gcc (Gentoo 7.3.0 p1.0) 7.3.0
(please get back to me for questions about the Gentoo patchset)

A disassembly of the binaries is attached to the glibc bug.
More info on request.