https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91373
Bug ID: 91373
Summary: gcc6.2.0: ((U32)((U16 * U16)) >> 31) cannot always get
correct result with gcc -O2
Product: gcc
Version: 6.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: qiang.fu at verisilicon dot com
Target Milestone: ---
Created attachment 46676
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46676&action=edit
main.c
Hi,
I met an issue with gcc6.2.0: ((U16 * U16) >> 31) cannot always get expected
result for gcc optimization switch '-O2'.
e.g.
(63139 * 36032) >> 30 = 0x2 // right
(63139 * 36032) >> 31 = 0x0 // wrong, should be 0x1.
I create a test case for it, see the attachement(main.c). It passed on gcc
4.6.3, but failed with gcc6.2.0 + release build(gcc -O2)
gcc -O2 main.c -o test
./test 63139 36032
data1: 0x879a1e40, data1 >> 31: 1, data1 >> 30: 2
data2: 0x879a1e40, data2 >> 31: 0, data1 >> 30: 2
This test passed if build with -O0 or -O1 with gcc6.2.0
GCC version: 6.2.0
*
gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/x86_64-pc-linux-gnu/6.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../configure -enable-checking=release -enable-languages=c,c++
-disable-multilib
Thread model: posix
gcc version 6.2.0 (GCC)
