https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65648
Bug ID: 65648
Summary: [5 Regression] Bad code due to IRA fails to recognize
the clobber in parallel
Product: gcc
Version: 5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: terry.guo at arm dot com
Created attachment 35200
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35200&action=edit
test case
When compile attached case with command:
arm-none-eabi-gcc -march=armv6-m -mthumb -Os x.c -S
The gcc generates bad code as below:
main:
push {r0, r1, r2, r4, r5, r6, r7, lr}
ldr r4, .L5
movs r5, #0
ldr r0, [r4, #8]
add r1, sp, #4
rsbs r2, r0, #0
adcs r2, r2, r0
subs r3, r2, r3 //r3 is used but not initialized
The instruction to initialize r3 is removed due to IRA failed to recognize the
clobber in reload pass. Before the reload pass, we have three instructions like
below:
(insn 12 11 14 2 (parallel [
(set (reg:SI 128)
(eq:SI (reg:SI 110 [ D.4914 ])
(const_int 0 [0])))
(clobber (reg:SI 129))
]) x.c:23 760 {*cstoresi_eq0_thumb1_insn}
(expr_list:REG_UNUSED (reg:SI 129)
(nil)))
(insn 20 19 22 2 (set (reg:SI 135)
(plus:SI (plus:SI (reg:SI 136)
(reg:SI 137))
(geu:SI (reg:SI 131 [ D.4914 ])
(reg:SI 134 [ c ])))) x.c:23 764 {thumb1_addsi3_addgeu}
(expr_list:REG_DEAD (reg:SI 137)
(expr_list:REG_DEAD (reg:SI 136)
(expr_list:REG_DEAD (reg:SI 134 [ c ])
(expr_list:REG_DEAD (reg:SI 131 [ D.4914 ])
(nil))))))
(insn 22 20 23 2 (set (reg:SI 138)
(minus:SI (reg:SI 128)
(reg:SI 135))) x.c:23 720 {thumb1_subsi3_insn}
(expr_list:REG_DEAD (reg:SI 135)
(expr_list:REG_DEAD (reg:SI 128)
(nil))))
After the reload pass, those instructions are wrongly turned into:
(insn 20 53 58 2 (set (reg:SI 3 r3 [135])
(plus:SI (plus:SI (reg:SI 3 r3 [137])
(reg:SI 2 r2 [136]))
(geu:SI (reg:SI 7 r7 [orig:131 D.4914 ] [131])
(reg:SI 6 r6 [153])))) x.c:23 764 {thumb1_addsi3_addgeu}
(nil))
(insn 58 20 55 2 (parallel [
(set (reg:SI 2 r2 [128])
(eq:SI (reg:SI 0 r0 [orig:110 D.4914 ] [110])
(const_int 0 [0])))
(clobber (reg:SI 3 r3 [129]))
]) x.c:23 760 {*cstoresi_eq0_thumb1_insn}
(nil))
(note 55 58 22 2 NOTE_INSN_DELETED)
(insn 22 55 23 2 (set (reg:SI 3 r3 [138])
(minus:SI (reg:SI 2 r2 [128])
(reg:SI 3 r3 [135]))) x.c:23 720 {thumb1_subsi3_insn}
(nil))
However the later pass can recognize the clobber in insn 58 and will remove the
insn 20 because the r3 defined in insn 20 is clobbered by insn 58.
