https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106167
Bug ID: 106167
Summary: Missed optimization (memory vs register read)
Product: gcc
Version: 9.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: vluchits at gmail dot com
Target Milestone: ---
Target: sh
Created attachment 53238
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53238&action=edit
Test case
Hello,
in the provided example, this piece of C code:
if (newclipbounds)
{
int newfloorclipx = floorclipx;
int newceilingclipx = ceilingclipx;
uint16_t newclip;
// rewrite clipbounds
if (actionbits & AC_NEWFLOOR)
newfloorclipx = low;
if (actionbits & AC_NEWCEILING)
newceilingclipx = high;
newclip = (newceilingclipx << 8) + newfloorclipx;
clipbounds[x] = newclip;
newclipbounds[x] = newclip;
}
is compiled to the following assembler code:
if (newclipbounds)
16c: 54 f7 mov.l @(28,r15),r4
16e: 24 48 tst r4,r4
170: 8d 10 bt.s 194 <_R_SegLoop+0x194>
172: e0 54 mov #84,r0
if (actionbits & AC_NEWFLOOR)
174: 05 fe mov.l @(r0,r15),r5
176: 25 58 tst r5,r5
178: 8f 01 bf.s 17e <_R_SegLoop+0x17e>
17a: e0 58 mov #88,r0
floorclipx = ceilingclipx & 0x00ff;
17c: 67 a3 mov r10,r7
if (actionbits & AC_NEWCEILING)
17e: 00 fe mov.l @(r0,r15),r0
180: 20 08 tst r0,r0
182: 8f 01 bf.s 188 <_R_SegLoop+0x188>
184: 50 fb mov.l @(44,r15),r0
int newceilingclipx = ceilingclipx;
186: 66 93 mov r9,r6
newclip = (newceilingclipx << 8) + newfloorclipx;
188: 46 18 shll8 r6
18a: 37 6c add r6,r7
18c: 67 7d extu.w r7,r7
clipbounds[x] = newclip;
18e: 08 75 mov.w r7,@(r0,r8)
newclipbounds[x] = newclip;
The following lines are of particular interest:
if (newclipbounds)
16c: 54 f7 mov.l @(28,r15),r4
16e: 24 48 tst r4,r4
...
newclipbounds[x] = newclip;
190: 50 f7 mov.l @(28,r15),r0
192: 08 75 mov.w r7,@(r0,r8)
The compiler fails to notice that it's already holding the value of @(28,r15)
in r4 and re-reads the value from stack instead of doing the mov r4,r0.
CFLAGS: -c -std=c11 -g -m2 -mb -Os -fomit-frame-pointer -Wall -Wextra -pedantic
-Wno-unused-parameter -Wimplicit-fallthrough=0 -Wno-missing-field-initializers
-Wnonnull
This may or may not be relevant to bug #106161
