[Bug c/65740] New: spectacularly bad inlinining decisions with -Os

westfw at yahoo dot com gcc-bugzilla@gcc.gnu.org
Sat Apr 11 05:38:00 GMT 2015


https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65740

            Bug ID: 65740
           Summary: spectacularly bad inlinining decisions with -Os
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: westfw at yahoo dot com

Created attachment 35298
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35298&action=edit
preprocessed source code example.

Compiling a module with 4.8.1 and -Os results in a binary more than triple the
size of compiling the same module with 4.3.2, due to bad decisions about
inlining a function.
Dumping inline debug info (-fdump-ipa-inline) shows that the compiler is coming
up with a negative "badness" for the especially-bad function (move3Servos), and
the number is "suspicious" (0xBFFFFFF8) (without understanding what the badness
is supposed to be.)

Considering void move3Servos(Servo, float, Servo, float, Servo, float, float)
with 87 size
 to be inlined into void loop() in oiOSoul.ino:325
 Estimated growth after inlined into all is +412 insns.
 Estimated badness is -1073741824, frequency 0.01.
 Inlined into void loop() which now has time 351 and size 628,net change of +0.


(also not that that was about the 40th call of move3Servos() from loop()
analyzed, each with "growth" ~400 insns, and it now thinks the total size is
628, which is ridiculous.)

Might be a duplicate of https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57218
Except that that seems to describe a more trivial and less awful ocurrence.

Originally noticed withing Arduino (avr-g++), but NOT cpu specific, and not C++
specific.  The attached example is x86 gcc 4.7.2:

/sw/lib/gcc4.7/bin/gcc-fsf-4.7 -c -Os -w foo.ii -v -saveemps -fdump-ipa-inline 
Using built-in specs.
COLLECT_GCC=/sw/lib/gcc4.7/bin/gcc-fsf-4.7
Target: x86_64-apple-darwin11.4.2
Configured with: ../gcc-4.7.2/configure --prefix=/sw --prefix=/sw/lib/gcc4.7
--mandir=/sw/share/man --infodir=/sw/lib/gcc4.7/info
--enable-languages=c,c++,fortran,lto,objc,obj-c++,java --with-gmp=/sw
--with-libiconv-prefix=/sw --with-ppl=/sw --with-cloog=/sw --with-mpc=/sw
--with-system-zlib --x-includes=/usr/X11R6/include --x-libraries=/usr/X11R6/lib
--program-suffix=-fsf-4.7 --enable-cloog-backend=isl
Thread model: posix
gcc version 4.7.2 (GCC) 
COLLECT_GCC_OPTIONS='-mmacosx-version-min=10.9.4' '-c' '-Os' '-w' '-v'
'-save-temps' '-fdump-ipa-inline' '-mtune=core2'
 /sw/lib/gcc4.7/libexec/gcc/x86_64-apple-darwin11.4.2/4.7.2/cc1plus
-fpreprocessed foo.ii -fPIC -quiet -dumpbase foo.ii -mmacosx-version-min=10.9.4
-mtune=core2 -auxbase foo -Os -w -version -fdump-ipa-inline -o foo.s
GNU C++ (GCC) version 4.7.2 (x86_64-apple-darwin11.4.2)
        compiled by GNU C version 4.7.2, GMP version 5.1.0, MPFR version 3.1.1,
MPC version 1.0.1
warning: GMP header version 5.1.0 differs from library version 5.1.1.
warning: MPFR header version 3.1.1 differs from library version 3.1.2.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C++ (GCC) version 4.7.2 (x86_64-apple-darwin11.4.2)
        compiled by GNU C version 4.7.2, GMP version 5.1.0, MPFR version 3.1.1,
MPC version 1.0.1
warning: GMP header version 5.1.0 differs from library version 5.1.1.
warning: MPFR header version 3.1.1 differs from library version 3.1.2.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 712698abe851e71eccb1a409a1351965
COLLECT_GCC_OPTIONS='-mmacosx-version-min=10.9.4' '-c' '-Os' '-w' '-v'
'-save-temps' '-fdump-ipa-inline' '-mtune=core2'
 as -arch x86_64 -force_cpusubtype_ALL -o foo.o foo.s
COMPILER_PATH=/sw/lib/gcc4.7/libexec/gcc/x86_64-apple-darwin11.4.2/4.7.2/:/sw/lib/gcc4.7/libexec/gcc/x86_64-apple-darwin11.4.2/4.7.2/:/sw/lib/gcc4.7/libexec/gcc/x86_64-apple-darwin11.4.2/:/sw/lib/gcc4.7/lib/gcc/x86_64-apple-darwin11.4.2/4.7.2/:/sw/lib/gcc4.7/lib/gcc/x86_64-apple-darwin11.4.2/
LIBRARY_PATH=/sw/lib/gcc4.7/lib/gcc/x86_64-apple-darwin11.4.2/4.7.2/:/sw/lib/gcc4.7/lib/gcc/x86_64-apple-darwin11.4.2/4.7.2/../../../:/usr/lib/
COLLECT_GCC_OPTIONS='-mmacosx-version-min=10.9.4' '-c' '-Os' '-w' '-v'
'-save-temps' '-fdump-ipa-inline' '-mtune=core2'
BillW-MacOSX-2<10422> size foo.o
__TEXT  __DATA  __OBJC  others  dec     hex
29936   209     0       0       30145   75c1

#(without inlining)
BillW-MacOSX-2<10423> /sw/lib/gcc4.7/bin/gcc-fsf-4.7 -c -Os -w foo.ii
-fno-inline-small-functions
BillW-MacOSX-2<10424> size foo.o
__TEXT  __DATA  __OBJC  others  dec     hex
8725    209     0       0       8934    22e6



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