https://llvm.org/bugs/show_bug.cgi?id=26445
Bug ID: 26445
Summary: [ppc] inefficient code generated for std::max(float,
float)
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Backend: PowerPC
Assignee: unassignedb...@nondot.org
Reporter: car...@google.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Compile following code with options: $ ~/llvm/obj2/bin/clang++
--target=powerpc64le-grtev4-linux-gnu -m64 -mvsx -mcpu=power8 -O2 -c -o t9.o
t9.cc -fno-unroll-loops
#include <algorithm>
float foo(float* input, int s) {
float max_value = input[0];
for (int j = 1; j <= s; ++j)
max_value = std::max(max_value, input[j]);
return max_value;
}
I got following code for the loop body
.LBB0_2: # %for.body
# =>This Inner Loop Header: Depth=1
lfsu 0, 4(3)
fcmpu 0, 1, 0
isel 5, 3, 4, 0
lwz 5, 0(5)
mtvsrd 34, 5
stw 5, -12(1)
xxsldwi 13, 34, 34, 1
xscvspdpn 1, 13
bdnz .LBB0_2
There are several problems in this code snippet
1. instead of compare and choose maximum float value, the generated code uses
isel to choose the address of larger value, then load it into integer register,
then move it to fp register, then expand it to double type.
2. no need to store the max value to memory.
It causes one of our internal applications more than 2x slower.
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