http://llvm.org/bugs/show_bug.cgi?id=22373
Bug ID: 22373
Summary: ARM: (sub ; trivial unsigned icmp ; branch) picks
wrong trivial cmp result
Product: tools
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: llc
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Given the following .ll
===
$ cat temp.repro2.ll
; ModuleID = '/tmp/temp.repro2.ll'
target datalayout = "e-p:32:32-i64:64-n32"
@x = internal global i32 zeroinitializer, align 4
define i32 @main() {
entry:
%bf.load = load i32* @x, align 1
%sub = sub i32 %bf.load, 2
%cmp = icmp ult i32 %sub, 0
br i1 %cmp, label %if.then, label %if.end
if.then: ; preds = %entry
call void @abort()
unreachable
if.end: ; preds = %entry
call void @exit(i32 0)
unreachable
}
declare void @abort()
declare void @exit(i32)
====
The icmp ult i32 %sub, 0, should always be false, so that the if.end branch is
taken.
In LLVM 3.5 we would get:
$ llc -mtriple armv7a-none-linux-gnueabihf /tmp/temp.repro2.ll -o -
main: @ @main
.fnstart
.Leh_func_begin0:
@ BB#0: @ %entry
.save {r11, lr}
push {r11, lr}
mov r0, #1
cmp r0, #0
bne .LBB0_2
@ BB#1: @ %if.then
bl abort
.LBB0_2: @ %if.end
mov r0, #0
bl exit
This correctly goes to the if.end branch.
I tried LLVM 3.7 @ 227294 and get:
main: @ @main
.fnstart
.Leh_func_begin0:
@ BB#0: @ %entry
.save {r11, lr}
push {r11, lr}
movw r0, :lower16:x
movt r0, :upper16:x
ldr r0, [r0]
subs r0, r0, #2
bhs .LBB0_2
@ BB#1: @ %if.then
bl abort
.LBB0_2: @ %if.end
mov r0, #0
bl exit
The subs will not actually set the carry bit, so this no longer goes to the
if.end branch.
====
Currently still bisecting to figure out when this started happening.
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