https://llvm.org/bugs/show_bug.cgi?id=23857
Bug ID: 23857
Summary: [AArch64] Sutraction interferes with umin generation
Product: tools
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: opt
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Created attachment 14478
--> https://llvm.org/bugs/attachment.cgi?id=14478&action=edit
IR reproducer
The following code will not generate umin/umax instructions because of the
subtract used to compute a.
void test(unsigned char *in0,
unsigned char *in1,
unsigned char *out,
unsigned n) {
unsigned i;
unsigned char a, b;
for (i = 0; i < n; i++)
{
a = 255 - in0[i]; // Replacing with a = in0[i] allows us to generate umin
b = in1[i];
out[i] = (unsigned char) (a < b ? a : b);
}
}
Reproduce with:
clang -cc1 -triple aarch64--linux-gnueabi test.c -S -o out.s -mllvm
-force-vector-width=16 -O3 -vectorize-loops
I've also attached a.ll which contains two similar functions obtained from this
code. This shows that removing the xor (sub) operation allows us to generate
umin:
opt -O3 aa.ll -o aa1.ll -S -force-vector-width=16
bin/llc -O3 aa1.ll -o aa1.s
Found on trunk (4a867c7a05bd0560cc16b3c16286e1c76073334e), Mon Jun 15 09:19:41
2015.
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