https://llvm.org/bugs/show_bug.cgi?id=28957
Bug ID: 28957
Summary: [x86] unnecessary mov of constant function parameter
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
I'm not sure which layer should handle this, but I'm assuming it's something
late in the backend:
extern int bar(int);
int foo(int x) {
if (x == 1)
return bar(1); // 1st parameter is already '1'
else
return bar(-1);
}
or as IR:
define i32 @foo(i32 %x) {
entry:
%cmp = icmp eq i32 %x, 1
br i1 %cmp, label %if.then, label %if.else
if.then:
%call = tail call i32 @bar(i32 1) #2
br label %return
if.else:
%call1 = tail call i32 @bar(i32 -1) #2
br label %return
return:
%retval.0 = phi i32 [ %call, %if.then ], [ %call1, %if.else ]
ret i32 %retval.0
}
declare i32 @bar(i32)
---------------------------------------------------------------------------
$ ./clang -O2 -fomit-frame-pointer repeatParameter.c -S -o -
...
cmpl $1, %edi
jne LBB0_2
## BB#1:
movl $1, %edi <--- %edi is already '1'
jmp _bar <--- 2nd bug? MachineCSE?
LBB0_2:
movl $-1, %edi
jmp _bar
gcc gets this right:
foo(int):
cmpl $1, %edi
je .L3
movl $-1, %edi
.L3:
jmp bar(int)
--
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