https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68086
Bug ID: 68086
Summary: Expression explicitly defined outside the loop is
moved inside the loop by the optimizer
Product: gcc
Version: 5.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: igusarov at mail dot ru
Target Milestone: ---
Created attachment 36578
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36578&action=edit
Single function to reproduce the results
Compileable C source in "ex324_core.c" does not include any header files. It
consists of a single function whose performance is spoiled by the optimizer.
Please read explanatory comments in that file.
"ex324.c" is a compileable test program build around the same core function. It
merely measures the amount of CPU clock ticks taken by that core function. It
includes system headers for printf and mmap, and is provided just for
convenience of testing.
The problem was first discovered in x86_64 gcc 5.2.0 compiler. Brief regression
research showed that 4.8.3 has this problem too. 4.7.4 seems to be good.
Problem in a nutshell. Let's start with this loop:
// Case 1
for (i = 0; i < size; ++i)
accumulator += data[i];
and rewrite it in this equivalent form:
// Case 2
int* rebased = data + size;
for (i = -size; i; ++i)
accumulator += rebased[i];
It looks like the forward propagation pass decides not to allocate a register
for variable 'rebased', but rather compute its value every time it is used in
the loop. This results in assembly output which, if written in terms of C,
would look like this:
for (i = -size; i; ++i)
accumulator += *(data + (size + i));
Extra operation inside the loop only slows the program down.
This happens at any optimization level above -O0.
Command line:
x86_64-unknown-freebsd9.0_5.2.0-gcc -O2 -S ex324_core.c
Compiler:
x86_64-unknown-freebsd9.0_5.2.0-gcc -v
Using built-in specs.
COLLECT_GCC=x86_64-unknown-freebsd9.0_5.2.0-gcc
COLLECT_LTO_WRAPPER=/usr/toolchain/x86_64-unknown-freebsd9.0_5.2.0/libexec/gcc/x86_64-unknown-freebsd9.0/5.2.0/lto-wrapper
Target: x86_64-unknown-freebsd9.0
Configured with:
/mnt/hdd/usr/home/toolbuilder/build_scripts/x86_64-unknown-freebsd9.0_5.2.0/build_scripts/../tools_build/x86_64-unknown-freebsd9.0_5.2.0/gcc-5.2.0/configure
--target=x86_64-unknown-freebsd9.0
--prefix=/usr/toolchain/x86_64-unknown-freebsd9.0_5.2.0
--with-local-prefix=/usr/local
--with-sysroot=/usr/toolchain/x86_64-unknown-freebsd9.0_5.2.0/sysroot
--program-prefix=x86_64-unknown-freebsd9.0_5.2.0- --with-gnu-as --with-gnu-ld
--with-as=/usr/toolchain/x86_64-unknown-freebsd9.0_5.2.0/bin/x86_64-unknown-freebsd9.0_5.2.0-as
--with-ld=/usr/toolchain/x86_64-unknown-freebsd9.0_5.2.0/bin/x86_64-unknown-freebsd9.0_5.2.0-ld
--with-nm=/usr/toolchain/x86_64-unknown-freebsd9.0_5.2.0/bin/x86_64-unknown-freebsd9.0_5.2.0-nm
--with-objdump=/usr/toolchain/x86_64-unknown-freebsd9.0_5.2.0/bin/x86_64-unknown-freebsd9.0_5.2.0-objdump
--with-gmp=/mnt/hdd/usr/home/toolbuilder/build_scripts/x86_64-unknown-freebsd9.0_5.2.0/build_scripts/../tools_build/x86_64-unknown-freebsd9.0_5.2.0/gmp-root
--with-mpfr=/mnt/hdd/usr/home/toolbuilder/build_scripts/x86_64-unknown-freebsd9.0_5.2.0/build_scripts/../tools_build/x86_64-unknown-freebsd9.0_5.2.0/mpfr-root
--with-mpc=/mnt/hdd/usr/home/toolbuilder/build_scripts/x86_64-unknown-freebsd9.0_5.2.0/build_scripts/../tools_build/x86_64-unknown-freebsd9.0_5.2.0/mpc-root
--disable-__cxa_atexit --enable-languages=c,c++ --disable-multilib
--disable-nls --enable-shared=libstdc++ --enable-static --enable-threads
Thread model: posix
gcc version 5.2.0 (GCC)
Operating system:
amd64 FreeBSD 9.0-RELEASE
CPU:
Intel(R) Core(TM) i7-2700K CPU @ 3.50GHz (3500.10-MHz K8-class CPU)
Origin = "GenuineIntel" Id = 0x206a7 Family = 6 Model = 2a Stepping = 7
Features=0xbfebfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,DTS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,PBE>
Features2=0x179ae3bf<SSE3,PCLMULQDQ,DTES64,MON,DS_CPL,VMX,EST,TM2,SSSE3,CX16,xTPR,PDCM,PCID,SSE4.1,SSE4.2,POPCNT,TSCDLT,AESNI,XSAVE,AVX>
AMD Features=0x28100800<SYSCALL,NX,RDTSCP,LM>
AMD Features2=0x1<LAHF>
