Remove a very old instcombine where we would turn sequences of selects into

logical operations on the i1's driving them.  This is a bad idea for every
target I can think of (confirmed with micro tests on all of: x86-64, ARM,
AArch64, Mips, and PowerPC) because it forces the i1 to be materialized into
a general purpose register, whereas consuming it directly into a select generally
allows it to exist only transiently in a predicate or flags register.

Chandler ran a set of performance tests with this change, and reported no
measurable change on x86-64.

llvm-svn: 201275

1 files changed, 24 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstCombine/select-select.ll b/llvm/test/Transforms/InstCombine/select-select.ll
new file mode 100644
index 00000000000..65820acf07b
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/select-select.ll
@@ -0,0 +1,24 @@
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+; CHECK: @foo1
+define float @foo1(float %a) #0 {
+; CHECK-NOT: xor
+  %b = fcmp ogt float %a, 0.000000e+00
+  %c = select i1 %b, float %a, float 0.000000e+00
+  %d = fcmp olt float %c, 1.000000e+00
+  %f = select i1 %d, float %c, float 1.000000e+00
+  ret float %f
+}
+
+; CHECK: @foo2
+define float @foo2(float %a) #0 {
+; CHECK-NOT: xor
+  %b = fcmp ogt float %a, 0.000000e+00
+  %c = select i1 %b, float %a, float 0.000000e+00
+  %d = fcmp olt float %c, 1.000000e+00
+  %e = select i1 %b, float %a, float 0.000000e+00
+  %f = select i1 %d, float %e, float 1.000000e+00
+  ret float %f
+}
+
+attributes #0 = { nounwind readnone ssp uwtable }