https://llvm.org/bugs/show_bug.cgi?id=27571
Bug ID: 27571
Summary: Codegen for checked addition is particularly poor
Product: libraries
Version: 3.7
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Backend: ARM
Assignee: unassignedb...@nondot.org
Reporter: dbau...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
The following C code results in code with a pile of seemingly unnecessary
instructions:
int checked(int a, int b) {
int res;
if (__builtin_sadd_overflow(a, b, &res)) {
__builtin_abort();
}
return res;
}
With `clang-3.7 -O3 -target arm-unknown-linux-gnueabihf -S
-fomit-frame-pointer` (the last is to just remove irrelevant cruft, it doesn't
change the core codegen) this gives:
checked: @ @checked
.fnstart
@ BB#0:
mov r2, r0
add r0, r2, r1
mov r1, #1
cmp r0, r2
movwvc r1, #0
cmp r1, #0
bxeq lr
mov lr, pc
b abort
The sequence `mov r1, #1` through `cmp r1 #0` seems particularly pointless: I
believe can be replaced by two instructions:
cmp r0, r2
bxvc lr
(I haven't found an arm variant with good codegen---v7 and v5 etc. are all
similar, and v6m is even worse!)
This seems to be particular to 32-bit ARM: x86 (64- and 32-bit) and AArch64
seem far better.
--
You are receiving this mail because:
You are on the CC list for the bug.
_______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
