https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109463
Bug ID: 109463
Summary: suboptimal sequence for converting 64-bit unsigned int
to float
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: elronnd at elronnd dot net
Target Milestone: ---
double f(uint64_t x) { return x; } gives:
test rdi,rdi
js 10 <f+0x10>
pxor xmm0,xmm0
cvtsi2sd xmm0,rdi
ret
nop
10:
mov rax,rdi
and edi,0x1
pxor xmm0,xmm0
shr rax,1
or rax,rdi
cvtsi2sd xmm0,rax
addsd xmm0,xmm0
ret
In particular, the sequence:
mov rax,rdi
and edi,0x1
shr rax,1
or rax,rdi
cvtsi2sd xmm0,rax
Can be replaced with:
movzx eax,dil
shr rdi,1
or rdi,rax
cvtsi2sd xmm0,rdi
Since all 9 low bits of rdi are below the sticky bit, oring them together in
any order suffices to round correctly.
Alternatively, in order to avoid clobbering rdi, use the following sequence:
mov rax,rdi
shr rax,1
or al,dil
cvtsi2sd xmm0,rax
(The penalty for partial register access appears to be very cheap or
nonexistent on recent uarchs.)
