https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69871
Bug ID: 69871
Summary: Type punned structs returned by value optimized poorly
Product: gcc
Version: 5.3.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: rf at rufflewind dot com
Target Milestone: ---
The following code, which unpacks a 32-bit integer into a struct of four bytes,
does not optimize as well as it should. While "unpack" seems to optimize just
fine, trivial wrappers of the function do not seem to get optimized nearly as
well:
- Two of the wrappers ("wrapper", "wrapper2") are completely identical yet they
do not result in the same assembly code. One is optimized well, the other is
not.
- Adding another layer of indirection ("wrapperwrapper") also prevents the
optimization from occurring.
The problem occurs not only for union-based type-punning, but also for similar
tricks that involve:
- memcpy, where all three wrappers would optimize poorly, or
- bitshift operators, where even "unpack" would optimize poorly.
See also: https://gcc.gnu.org/ml/gcc/2016-02/msg00244.html
The code was compiled with "gcc -fverbose-asm -Wall -S -O3 foo.c" on Linux
4.4.1 x86-64. The GCC binaries are part of the Arch Linux's gcc-multilib
5.3.0-4 binary package.
---
struct alpha {
char a, b, c, d;
};
struct alpha unpack(unsigned x)
{
union {
struct alpha r;
unsigned i;
} u;
u.i = x;
return u.r;
}
struct alpha wrapper(unsigned y)
{
return unpack(y);
}
struct alpha wrapper2(unsigned y)
{
return unpack(y);
}
struct alpha wrapperwrapper(unsigned y)
{
return wrapper(y);
}
---
.file "foo.c"
# GNU C11 (GCC) version 5.3.0 (x86_64-unknown-linux-gnu)
# compiled by GNU C version 5.3.0, GMP version 6.1.0, MPFR version
3.1.3-p5, MPC version 1.0.3
# GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
# options passed: foo.c -mtune=generic -march=x86-64 -O3 -Wall
# -fverbose-asm
# options enabled: -faggressive-loop-optimizations -falign-labels
# -fasynchronous-unwind-tables -fauto-inc-dec -fbranch-count-reg
# -fcaller-saves -fchkp-check-incomplete-type -fchkp-check-read
# -fchkp-check-write -fchkp-instrument-calls -fchkp-narrow-bounds
# -fchkp-optimize -fchkp-store-bounds -fchkp-use-static-bounds
# -fchkp-use-static-const-bounds -fchkp-use-wrappers
# -fcombine-stack-adjustments -fcommon -fcompare-elim -fcprop-registers
# -fcrossjumping -fcse-follow-jumps -fdefer-pop
# -fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively
# -fdwarf2-cfi-asm -fearly-inlining -feliminate-unused-debug-types
# -fexpensive-optimizations -fforward-propagate -ffunction-cse -fgcse
# -fgcse-after-reload -fgcse-lm -fgnu-runtime -fgnu-unique
# -fguess-branch-probability -fhoist-adjacent-loads -fident -fif-conversion
# -fif-conversion2 -findirect-inlining -finline -finline-atomics
# -finline-functions -finline-functions-called-once
# -finline-small-functions -fipa-cp -fipa-cp-alignment -fipa-cp-clone
# -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-profile
# -fipa-pure-const -fipa-ra -fipa-reference -fipa-sra -fira-hoist-pressure
# -fira-share-save-slots -fira-share-spill-slots
# -fisolate-erroneous-paths-dereference -fivopts -fkeep-static-consts
# -fleading-underscore -flifetime-dse -flra-remat -flto-odr-type-merging
# -fmath-errno -fmerge-constants -fmerge-debug-strings
# -fmove-loop-invariants -fomit-frame-pointer -foptimize-sibling-calls
# -foptimize-strlen -fpartial-inlining -fpeephole -fpeephole2
# -fpredictive-commoning -fprefetch-loop-arrays -free -freg-struct-return
# -freorder-blocks -freorder-blocks-and-partition -freorder-functions
# -frerun-cse-after-loop -fsched-critical-path-heuristic
# -fsched-dep-count-heuristic -fsched-group-heuristic -fsched-interblock
# -fsched-last-insn-heuristic -fsched-rank-heuristic -fsched-spec
# -fsched-spec-insn-heuristic -fsched-stalled-insns-dep -fschedule-fusion
# -fschedule-insns2 -fsemantic-interposition -fshow-column -fshrink-wrap
# -fsigned-zeros -fsplit-ivs-in-unroller -fsplit-wide-types -fssa-phiopt
# -fstdarg-opt -fstrict-aliasing -fstrict-overflow
# -fstrict-volatile-bitfields -fsync-libcalls -fthread-jumps
# -ftoplevel-reorder -ftrapping-math -ftree-bit-ccp -ftree-builtin-call-dce
# -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
# -ftree-copyrename -ftree-cselim -ftree-dce -ftree-dominator-opts
# -ftree-dse -ftree-forwprop -ftree-fre -ftree-loop-distribute-patterns
# -ftree-loop-if-convert -ftree-loop-im -ftree-loop-ivcanon
# -ftree-loop-optimize -ftree-loop-vectorize -ftree-parallelize-loops=
# -ftree-partial-pre -ftree-phiprop -ftree-pre -ftree-pta -ftree-reassoc
# -ftree-scev-cprop -ftree-sink -ftree-slp-vectorize -ftree-slsr -ftree-sra
# -ftree-switch-conversion -ftree-tail-merge -ftree-ter -ftree-vrp
# -funit-at-a-time -funswitch-loops -funwind-tables -fverbose-asm
# -fzero-initialized-in-bss -m128bit-long-double -m64 -m80387
# -malign-stringops -mavx256-split-unaligned-load
# -mavx256-split-unaligned-store -mfancy-math-387 -mfp-ret-in-387 -mfxsr
# -mglibc -mieee-fp -mlong-double-80 -mmmx -mno-sse4 -mpush-args -mred-zone
# -msse -msse2 -mtls-direct-seg-refs -mvzeroupper
.section .text.unlikely,"ax",@progbits
.LCOLDB0:
.text
.LHOTB0:
.p2align 4,,15
.globl unpack
.type unpack, @function
unpack:
.LFB0:
.cfi_startproc
movl %edi, %eax # x, x
ret
.cfi_endproc
.LFE0:
.size unpack, .-unpack
.section .text.unlikely
.LCOLDE0:
.text
.LHOTE0:
.section .text.unlikely
.LCOLDB1:
.text
.LHOTB1:
.p2align 4,,15
.globl wrapper
.type wrapper, @function
wrapper:
.LFB5:
.cfi_startproc
movl %edi, %eax # y, y
xorl %edx, %edx # retval.9
movsbl %ah, %eax # y, SR.14
movb %dil, %dl # y, retval.9
movb %al, %dh # SR.14, retval.9
movl %edi, %eax # y, tmp101
andl $-16777216, %edi #, tmp105
andl $16711680, %eax #, tmp101
movzwl %dx, %edx # retval.9, tmp103
orl %eax, %edx # tmp101, tmp106
movl %edx, %eax # tmp106, tmp107
orl %edi, %eax # tmp105, tmp107
ret
.cfi_endproc
.LFE5:
.size wrapper, .-wrapper
.section .text.unlikely
.LCOLDE1:
.text
.LHOTE1:
.section .text.unlikely
.LCOLDB2:
.text
.LHOTB2:
.p2align 4,,15
.globl wrapper2
.type wrapper2, @function
wrapper2:
.LFB2:
.cfi_startproc
movl %edi, %eax # y, y
ret
.cfi_endproc
.LFE2:
.size wrapper2, .-wrapper2
.section .text.unlikely
.LCOLDE2:
.text
.LHOTE2:
.section .text.unlikely
.LCOLDB3:
.text
.LHOTB3:
.p2align 4,,15
.globl wrapperwrapper
.type wrapperwrapper, @function
wrapperwrapper:
.LFB3:
.cfi_startproc
movl %edi, %eax # y, y
xorl %edx, %edx # D.1859
movsbl %ah, %eax # y, SR.5
movb %dil, %dl # y, D.1859
movb %al, %dh # SR.5, D.1859
movl %edi, %eax # y, tmp101
andl $-16777216, %edi #, tmp105
andl $16711680, %eax #, tmp101
movzwl %dx, %edx # D.1859, tmp103
orl %eax, %edx # tmp101, tmp106
movl %edx, %eax # tmp106, tmp107
orl %edi, %eax # tmp105, tmp107
ret
.cfi_endproc
.LFE3:
.size wrapperwrapper, .-wrapperwrapper
.section .text.unlikely
.LCOLDE3:
.text
.LHOTE3:
.ident "GCC: (GNU) 5.3.0"
.section .note.GNU-stack,"",@progbits
