[ARM] Lower (select_cc k k (select_cc ~k ~k x)) into (SSAT l_k x)

Summary:
SSAT saturates an integer, making sure that its value lies within
an interval [-k, k]. Since the constant is given to SSAT as the
number of bytes set to one, k + 1 must be a power of 2, otherwise
the optimization is not possible. Also, the select_cc must use <
and > respectively so that they define an interval.

Reviewers: mcrosier, jmolloy, rengolin

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D21372

llvm-svn: 273581

1 files changed, 215 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/ARM/ssat.ll b/llvm/test/CodeGen/ARM/ssat.ll
new file mode 100644
index 00000000000..2b75bc410aa
--- /dev/null
+++ b/llvm/test/CodeGen/ARM/ssat.ll
@@ -0,0 +1,215 @@
+; RUN: llc -mtriple=arm-eabi %s -o - | FileCheck %s
+
+; Check for several conditions that should result in SSAT.
+; For example, the base test is equivalent to
+; x < -k ? -k : (x > k ? k : x) in C. All patterns that bound x
+; to the interval [-k, k] where k is a power of 2 can be
+; transformed into SSAT. At the end there are some tests
+; checking that conditionals are not transformed if they don't
+; match the right pattern.
+
+;
+; Base tests with different bit widths
+;
+
+; x < -k ? -k : (x > k ? k : x)
+; 32-bit base test
+define i32 @sat_base_32bit(i32 %x) #0 {
+; CHECK-LABEL: sat_base_32bit:
+; CHECK: ssat r0, #24, r0
+entry:
+  %cmpLow = icmp slt i32 %x, -8388608
+  %cmpUp = icmp sgt i32 %x, 8388607
+  %saturateUp = select i1 %cmpUp, i32 8388607, i32 %x
+  %saturateLow = select i1 %cmpLow, i32 -8388608, i32 %saturateUp
+  ret i32 %saturateLow
+}
+
+; x < -k ? -k : (x > k ? k : x)
+; 16-bit base test
+define i16 @sat_base_16bit(i16 %x) #0 {
+; CHECK-LABEL: sat_base_16bit:
+; CHECK: ssat r0, #12, r0
+entry:
+  %cmpLow = icmp slt i16 %x, -2048
+  %cmpUp = icmp sgt i16 %x, 2047
+  %saturateUp = select i1 %cmpUp, i16 2047, i16 %x
+  %saturateLow = select i1 %cmpLow, i16 -2048, i16 %saturateUp
+  ret i16 %saturateLow
+}
+
+; x < -k ? -k : (x > k ? k : x)
+; 8-bit base test
+define i8 @sat_base_8bit(i8 %x) #0 {
+; CHECK-LABEL: sat_base_8bit:
+; CHECK: ssat r0, #6, r0
+entry:
+  %cmpLow = icmp slt i8 %x, -32
+  %cmpUp = icmp sgt i8 %x, 31
+  %saturateUp = select i1 %cmpUp, i8 31, i8 %x
+  %saturateLow = select i1 %cmpLow, i8 -32, i8 %saturateUp
+  ret i8 %saturateLow
+}
+
+;
+; Tests where the conditionals that check for upper and lower bounds,
+; or the < and > operators, are arranged in different ways. Only some
+; of the possible combinations that lead to SSAT are tested.
+;
+
+; x < -k ? -k : (x < k ? x : k)
+define i32 @sat_lower_upper_1(i32 %x) #0 {
+; CHECK-LABEL: sat_lower_upper_1:
+; CHECK: ssat r0, #24, r0
+entry:
+  %cmpLow = icmp slt i32 %x, -8388608
+  %cmpUp = icmp slt i32 %x, 8388607
+  %saturateUp = select i1 %cmpUp, i32 %x, i32 8388607
+  %saturateLow = select i1 %cmpLow, i32 -8388608, i32 %saturateUp
+  ret i32 %saturateLow
+}
+
+; x > -k ? (x > k ? k : x) : -k
+define i32 @sat_lower_upper_2(i32 %x) #0 {
+; CHECK-LABEL: sat_lower_upper_2:
+; CHECK: ssat    r0, #24, r0
+entry:
+  %cmpLow = icmp sgt i32 %x, -8388608
+  %cmpUp = icmp sgt i32 %x, 8388607
+  %saturateUp = select i1 %cmpUp, i32 8388607, i32 %x
+  %saturateLow = select i1 %cmpLow, i32 %saturateUp, i32 -8388608
+  ret i32 %saturateLow
+}
+
+; x < k ? (x < -k ? -k : x) : k
+define i32 @sat_upper_lower_1(i32 %x) #0 {
+; CHECK-LABEL: sat_upper_lower_1:
+; CHECK: ssat    r0, #24, r0
+entry:
+  %cmpUp = icmp slt i32 %x, 8388607
+  %cmpLow = icmp slt i32 %x, -8388608
+  %saturateLow = select i1 %cmpLow, i32 -8388608, i32 %x
+  %saturateUp = select i1 %cmpUp, i32 %saturateLow, i32 8388607
+  ret i32 %saturateUp
+}
+
+; x > k ? k : (x < -k ? -k : x)
+define i32 @sat_upper_lower_2(i32 %x) #0 {
+; CHECK-LABEL: sat_upper_lower_2:
+; CHECK: ssat    r0, #24, r0
+entry:
+  %cmpUp = icmp sgt i32 %x, 8388607
+  %cmpLow = icmp slt i32 %x, -8388608
+  %saturateLow = select i1 %cmpLow, i32 -8388608, i32 %x
+  %saturateUp = select i1 %cmpUp, i32 8388607, i32 %saturateLow
+  ret i32 %saturateUp
+}
+
+; k < x ? k : (x > -k ? x : -k)
+define i32 @sat_upper_lower_3(i32 %x) #0 {
+; CHECK-LABEL: sat_upper_lower_3:
+; CHECK: ssat    r0, #24, r0
+entry:
+  %cmpUp = icmp slt i32 8388607, %x
+  %cmpLow = icmp sgt i32 %x, -8388608
+  %saturateLow = select i1 %cmpLow, i32 %x, i32 -8388608
+  %saturateUp = select i1 %cmpUp, i32 8388607, i32 %saturateLow
+  ret i32 %saturateUp
+}
+
+;
+; Miscellanea
+;
+
+; Check that >= and <= work the same as > and <
+; k <= x ? k : (x >= -k ? x : -k)
+define i32 @sat_le_ge(i32 %x) #0 {
+; CHECK-LABEL: sat_le_ge:
+; CHECK: ssat    r0, #24, r0
+entry:
+  %cmpUp = icmp sle i32 8388607, %x
+  %cmpLow = icmp sge i32 %x, -8388608
+  %saturateLow = select i1 %cmpLow, i32 %x, i32 -8388608
+  %saturateUp = select i1 %cmpUp, i32 8388607, i32 %saturateLow
+  ret i32 %saturateUp
+}
+
+;
+; The following tests check for patterns that should not transform
+; into SSAT but are similar enough that could confuse the selector.
+;
+
+; x > k ? k : (x > -k ? -k : x)
+; First condition upper-saturates, second doesn't lower-saturate.
+define i32 @no_sat_missing_lower(i32 %x) #0 {
+; CHECK-LABEL: no_sat_missing_lower
+; CHECK-NOT: ssat
+entry:
+  %cmpUp = icmp sgt i32 %x, 8388607
+  %cmpLow = icmp sgt i32 %x, -8388608
+  %saturateLow = select i1 %cmpLow, i32 -8388608, i32 %x
+  %saturateUp = select i1 %cmpUp, i32 8388607, i32 %saturateLow
+  ret i32 %saturateUp
+}
+
+; x < k ? k : (x < -k ? -k : x)
+; Second condition lower-saturates, first doesn't upper-saturate.
+define i32 @no_sat_missing_upper(i32 %x) #0 {
+; CHECK-LABEL: no_sat_missing_upper:
+; CHECK-NOT: ssat
+entry:
+  %cmpUp = icmp slt i32 %x, 8388607
+  %cmpLow = icmp slt i32 %x, -8388608
+  %saturateLow = select i1 %cmpLow, i32 -8388608, i32 %x
+  %saturateUp = select i1 %cmpUp, i32 8388607, i32 %saturateLow
+  ret i32 %saturateUp
+}
+
+; Lower constant is different in the select and in the compare
+define i32 @no_sat_incorrect_constant(i32 %x) #0 {
+; CHECK-LABEL: no_sat_incorrect_constant:
+; CHECK-NOT: ssat
+entry:
+  %cmpUp = icmp sgt i32 %x, 8388607
+  %cmpLow = icmp slt i32 %x, -8388608
+  %saturateLow = select i1 %cmpLow, i32 -8388607, i32 %x
+  %saturateUp = select i1 %cmpUp, i32 8388607, i32 %saturateLow
+  ret i32 %saturateUp
+}
+
+; The interval is not [k, ~k]
+define i32 @no_sat_incorrect_interval(i32 %x) #0 {
+; CHECK-LABEL: no_sat_incorrect_interval:
+; CHECK-NOT: ssat
+entry:
+  %cmpUp = icmp sgt i32 %x, 8388607
+  %cmpLow = icmp slt i32 %x, -19088744
+  %saturateLow = select i1 %cmpLow, i32 -19088744, i32 %x
+  %saturateUp = select i1 %cmpUp, i32 8388607, i32 %saturateLow
+  ret i32 %saturateUp
+}
+
+; The returned value (y) is not the same as the tested value (x).
+define i32 @no_sat_incorrect_return(i32 %x, i32 %y) #0 {
+; CHECK-LABEL: no_sat_incorrect_return:
+; CHECK-NOT: ssat
+entry:
+  %cmpUp = icmp sgt i32 %x, 8388607
+  %cmpLow = icmp slt i32 %x, -8388608
+  %saturateLow = select i1 %cmpLow, i32 -8388608, i32 %y
+  %saturateUp = select i1 %cmpUp, i32 8388607, i32 %saturateLow
+  ret i32 %saturateUp
+}
+
+; One of the values in a compare (y) is not the same as the rest
+; of the compare and select values (x).
+define i32 @no_sat_incorrect_compare(i32 %x, i32 %y) #0 {
+; CHECK-LABEL: no_sat_incorrect_compare:
+; CHECK-NOT: ssat
+entry:
+  %cmpUp = icmp sgt i32 %x, 8388607
+  %cmpLow = icmp slt i32 %y, -8388608
+  %saturateLow = select i1 %cmpLow, i32 -8388608, i32 %x
+  %saturateUp = select i1 %cmpUp, i32 8388607, i32 %saturateLow
+  ret i32 %saturateUp
+}