https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67691
Bug ID: 67691
Summary: [SH] Omit zero extension of shift amounds for dynamic
shifts
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: olegendo at gcc dot gnu.org
Target Milestone: ---
Target: sh*-*-*
In CSiBE I've seen code that goes like this ...
left shift:
mov.b @(...),r0
extu.b r0,r1
...
shld r1,r2
right shift:
mov.b @(...),r0
extu.b r0,r1
neg r1,r1
...
shld r1,r2
In both cases the zero extension can be omitted. The shld/shad shift count is
truncated to a 5 bit and as per C/C++ standard shift counts > 31 or negative
shift counts are undefined behavior for int32_t/uint32_t.
If a shift count value is loaded from memory and is not explicitly AND'ed with
31, it can be assumed that the value is in the range 0..31.
For example
value in mem signed ext. unsigned ext.
0...31 0...31 0...31 OK
33 33 33 NG
255 255 -1 NG
I can't think of a case where keeping the zero extension would avoid undefined
behavior.
