[Bug tree-optimization/15533] New: Missed move to partial register

["cesarb at nitnet dot com dot br" <gcc-bugzilla at gcc dot gnu dot org>]

When compiling:
#include <stdint.h>
#define regparm __attribute__((regparm(3)))
uint8_t a;
uint16_t regparm fn(uint16_t b)
{ return (b & ~0xFF) | a; }

With:
gcc -W -Wall -O3 -ffast-math -fomit-frame-pointer -fexpensive-optimizations
-save-temps -march=athlon -c

The result is:
	.file	"testcase.c"
	.text
	.align 4
	.p2align 4,,15
.globl fn
	.type	fn, @function
fn:
	movzbw	a, %dx
	xorb	%al, %al
	orl	%edx, %eax
	movzwl	%ax, %eax
	ret
	.size	fn, .-fn
	.comm	a,1,1
	.ident	"GCC: (GNU) 3.5.0 20040502 (experimental)"
	.section	.note.GNU-stack,"",@progbits

The expected result would be:
fn:
	movb a, %al
	ret

(I assume the correct ABI for regparm would be for the caller to do all the
sign/zero extension work, since it could be done as side effects of calculations
there, so there is no need to do any extension inside fn. If I'm wrong, just add
an extra movzwl	%ax, %eax somewhere)

Reading specs from /usr/lib/gcc-snapshot/lib/gcc/i486-linux/3.5.0/specs
Configured with: ../src/configure -v
--enable-languages=c,c++,java,f77,objc,ada,treelang
--prefix=/usr/lib/gcc-snapshot --enable-shared --with-system-zlib --enable-nls
--enable-threads=posix --without-included-gettext --disable-werror
--enable-__cxa_atexit --enable-clocale=gnu --enable-libstdcxx-debug
--enable-java-gc=boehm --enable-java-awt=gtk i486-linux
Thread model: posix
gcc version 3.5.0 20040502 (experimental)

-- 
           Summary: Missed move to partial register
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: cesarb at nitnet dot com dot br
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15533