https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99011
Bug ID: 99011
Summary: Potentially missed optimization. Arrays are created
without need
Product: gcc
Version: 10.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: pj at hugeone dot co.uk
Target Milestone: ---
Consider the code:
```
int bar(int N)
{
return N > 10 ? 14 : 8;
}
int foo(int N)
{
return (const int[]){8, 14}[N > 10];
}
int zoo(int N)
{
int a[] = {8,14};
return a[N > 10];
}
int boo(int N)
{
const static int a[] = {8,14};
return a[N > 10];
}
```
IMO in all cases the generated code should be the same as generated for
function bar. IMO arrays can be optimized out.
Compiled with -O3
foo:
mov rax, QWORD PTR .LC0[rip]
mov QWORD PTR [rsp-8], rax
xor eax, eax
cmp edi, 10
setgal
mov eax, DWORD PTR [rsp-8+rax*4]
ret
bar:
cmp edi, 10
mov edx, 8
mov eax, 14
cmovle eax, edx
ret
zoo:
mov rax, QWORD PTR .LC0[rip]
mov QWORD PTR [rsp-8], rax
xor eax, eax
cmp edi, 10
setgal
mov eax, DWORD PTR [rsp-8+rax*4]
ret
boo:
xor eax, eax
cmp edi, 10
setgal
mov eax, DWORD PTR a.0[0+rax*4]
ret
a.0:
.long 8
.long 14
.LC0:
.long 8
.long 14
gcc -O3 --verbose:
Using built-in specs.
COLLECT_GCC=/opt/compiler-explorer/gcc-10.2.0/bin/gcc
Target: x86_64-linux-gnu
Configured with: ../gcc-10.2.0/configure
--prefix=/opt/compiler-explorer/gcc-build/staging --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu --disable-bootstrap
--enable-multiarch --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --enable-clocale=gnu
--enable-languages=c,c++,fortran,ada,go,d --enable-ld=yes --enable-gold=yes
--enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-linker-build-id
--enable-lto --enable-plugins --enable-threads=posix
--with-pkgversion=Compiler-Explorer-Build
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 10.2.0 (Compiler-Explorer-Build)
COLLECT_GCC_OPTIONS='-fdiagnostics-color=always' '-g' '-o' './output.s'
'-masm=intel' '-S' '-v' '-O3' '-mtune=generic' '-march=x86-64'
/opt/compiler-explorer/gcc-10.2.0/bin/../libexec/gcc/x86_64-linux-gnu/10.2.0/cc1
-quiet -v -imultiarch x86_64-linux-gnu -iprefix
/opt/compiler-explorer/gcc-10.2.0/bin/../lib/gcc/x86_64-linux-gnu/10.2.0/
-quiet -dumpbase example.c -masm=intel -mtune=generic -march=x86-64
-auxbase-strip ./output.s -g -O3 -version -fdiagnostics-color=always -o
./output.s
GNU C17 (Compiler-Explorer-Build) version 10.2.0 (x86_64-linux-gnu)
compiled by GNU C version 7.5.0, GMP version 6.1.0, MPFR version 3.1.4,
MPC version 1.0.3, isl version isl-0.18-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory
"/opt/compiler-explorer/gcc-10.2.0/bin/../lib/gcc/x86_64-linux-gnu/10.2.0/../../../../x86_64-linux-gnu/include"
ignoring duplicate directory
"/opt/compiler-explorer/gcc-10.2.0/bin/../lib/gcc/../../lib/gcc/x86_64-linux-gnu/10.2.0/include"
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring duplicate directory
"/opt/compiler-explorer/gcc-10.2.0/bin/../lib/gcc/../../lib/gcc/x86_64-linux-gnu/10.2.0/include-fixed"
ignoring nonexistent directory
"/opt/compiler-explorer/gcc-10.2.0/bin/../lib/gcc/../../lib/gcc/x86_64-linux-gnu/10.2.0/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/opt/compiler-explorer/gcc-10.2.0/bin/../lib/gcc/x86_64-linux-gnu/10.2.0/include
/opt/compiler-explorer/gcc-10.2.0/bin/../lib/gcc/x86_64-linux-gnu/10.2.0/include-fixed
/usr/local/include
/opt/compiler-explorer/gcc-10.2.0/bin/../lib/gcc/../../include
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
GNU C17 (Compiler-Explorer-Build) version 10.2.0 (x86_64-linux-gnu)
compiled by GNU C version 7.5.0, GMP version 6.1.0, MPFR version 3.1.4,
MPC version 1.0.3, isl version isl-0.18-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 870fa0f1a93c5f0ff6fd5ef23d839e5a
COMPILER_PATH=/opt/compiler-explorer/gcc-10.2.0/bin/../libexec/gcc/x86_64-linux-gnu/10.2.0/:/opt/compiler-explorer/gcc-10.2.0/bin/../libexec/gcc/x86_64-linux-gnu/:/opt/compiler-explorer/gcc-10.2.0/bin/../libexec/gcc/:/opt/compiler-explorer/gcc-10.2.0/bin/../lib/gcc/x86_64-linux-gnu/10.2.0/../../../../x86_64-linux-gnu/bin/
