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[Bug rtl-optimization/55583] New: Extended shift instruction on x86-64 is not used, producing unoptimal code
- From: "mtkilpailut at torni dot org" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Tue, 04 Dec 2012 00:06:00 +0000
- Subject: [Bug rtl-optimization/55583] New: Extended shift instruction on x86-64 is not used, producing unoptimal code
- Auto-submitted: auto-generated
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55583
Bug #: 55583
Summary: Extended shift instruction on x86-64 is not used,
producing unoptimal code
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: rtl-optimization
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: mtkilpailut@torni.org
Created attachment 28866
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28866
Source code demonstrating bad code generation
On x86-64, extended shift instruction is not generated for some reason.
Combined with other problems this creates very bad code.
Test functions included for signed and unsigned 16,32,64-bit types for both
left and right shifts and for constant n and function parameter n.
Code of this form:
unsigned int a, b; const int n = 2;
void test32l (void) { b = (b << n) | (a >> (32 - n)); }
expected code:
mov a(%rip),%eax
shld $0x2,%eax,b(%rip)
ret
produced code:
mov b(%rip), %edx ; Size of register used here depends on gcc version
mov a(%rip), %eax ; Size of register used here depends on gcc version
sal $2, %edx ; Size of register used here depends on gcc version
shr $25, %eax
or %edx, %eax
mov %eax, b(%rip)
ret
Tested with:
COLLECT_GCC_OPTIONS='-v' '-c' '-save-temps' '-O2' '-Wall' '-W' '-o'
'gcc_shld_not_used' '-mtune=generic'
I tried gcc versions:
GNU C (Debian 4.7.2-4) version 4.7.2 (x86_64-linux-gnu)
GNU C (Debian 4.6.3-11) version 4.6.3 (x86_64-linux-gnu)
GNU C (Debian 4.5.3-9) version 4.5.3 (x86_64-linux-gnu)
GNU C (Debian 4.4.7-2) version 4.4.7 (x86_64-linux-gnu)
GNU C (GCC) version 4.8.0 20121203 (experimental) [trunk revision 194106]
(x86_64-unknown-linux-gnu)
All produce the same code modulo register size differences mentioned above. gcc
HEAD changes sal to leal (,%rcx,4),%eax