| Summary: | writing all bitfields in a word | ||
|---|---|---|---|
| Product: | gcc | Reporter: | jreiser |
| Component: | rtl-optimization | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | RESOLVED DUPLICATE | ||
| Severity: | enhancement | CC: | gcc-bugs |
| Priority: | P3 | Keywords: | missed-optimization |
| Version: | 3.2 | ||
| Target Milestone: | --- | ||
| Host: | Target: | ||
| Build: | Known to work: | ||
| Known to fail: | Last reconfirmed: | 2003-11-08 23:31:54 | |
| Attachments: | bitfield.i | ||
confirmed that it still happends on mainline (20030524). |
There is an opportunity for improved optimization when writing all the bitfields in a word ('int', 'unsigned', or 'long'): construct the entire word result in a register, then perform only one store to memory, instead of writing the fields to memory separately in turn. The attached bitfield.i generates (with "-O" optimization) movl (%ebx), %ecx andl $16777215, %ecx movl (%edx), %eax andl $-16777216, %eax orl %ecx, %eax movl %eax, (%edx) movzbl 3(%ebx), %ecx andb $127, %cl movzbl 3(%edx), %eax andb $-128, %al orb %cl, %al orb $-128, %al movb %al, 3(%edx) which could be improved to movl (%ebx), %eax orl $1<<31, %eax movl %eax, (%edx) which is much shorter and faster. Release: gcc version 3.2 Environment: Reading specs from /usr/local/gcc-3.2/lib/gcc-lib/i686-pc-linux-gnu/3.2/specs Configured with: ../gcc-3.2/configure --prefix=/usr/local/gcc-3.2 Thread model: posix gcc version 3.2 How-To-Repeat: gcc -v -save-temps -O -S bitfield.c