Canonicalize min/max expressions correctly.

This patch introduces a canonical form for min/max idioms where one operand
is extended or truncated. This often happens when the other operand is a
constant. For example:

  %1 = icmp slt i32 %a, i32 0
  %2 = sext i32 %a to i64
  %3 = select i1 %1, i64 %2, i64 0

Would now be canonicalized into:

  %1 = icmp slt i32 %a, i32 0
  %2 = select i1 %1, i32 %a, i32 0
  %3 = sext i32 %2 to i64

This builds upon a patch posted by David Majenemer
(https://www.marc.info/?l=llvm-commits&m=143008038714141&w=2). That pass
passively stopped instcombine from ruining canonical patterns. This
patch additionally actively makes instcombine canonicalize too.

Canonicalization of expressions involving a change in type from int->fp
or fp->int are not yet implemented.

llvm-svn: 237453

1 files changed, 73 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstCombine/minmax-fold.ll b/llvm/test/Transforms/InstCombine/minmax-fold.ll
new file mode 100644
index 00000000000..f50370713ff
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/minmax-fold.ll
@@ -0,0 +1,73 @@
+; RUN: opt -S -instcombine < %s | FileCheck %s
+
+; CHECK-LABEL: @t1
+; CHECK-NEXT: icmp
+; CHECK-NEXT: select
+; CHECK-NEXT: sext
+define i64 @t1(i32 %a) {
+  ; This is the canonical form for a type-changing min/max.
+  %1 = icmp slt i32 %a, 5
+  %2 = select i1 %1, i32 %a, i32 5
+  %3 = sext i32 %2 to i64
+  ret i64 %3
+}
+
+; CHECK-LABEL: @t2
+; CHECK-NEXT: icmp
+; CHECK-NEXT: select
+; CHECK-NEXT: sext
+define i64 @t2(i32 %a) {
+  ; Check this is converted into canonical form, as above.
+  %1 = icmp slt i32 %a, 5
+  %2 = sext i32 %a to i64
+  %3 = select i1 %1, i64 %2, i64 5
+  ret i64 %3
+}
+
+; CHECK-LABEL: @t3
+; CHECK-NEXT: icmp
+; CHECK-NEXT: select
+; CHECK-NEXT: zext
+define i64 @t3(i32 %a) {
+  ; Same as @t2, with flipped operands and zext instead of sext.
+  %1 = icmp ult i32 %a, 5
+  %2 = zext i32 %a to i64
+  %3 = select i1 %1, i64 5, i64 %2
+  ret i64 %3
+}
+
+; CHECK-LABEL: @t4
+; CHECK-NEXT: icmp
+; CHECK-NEXT: select
+; CHECK-NEXT: trunc
+define i32 @t4(i64 %a) {
+  ; Same again, with trunc.
+  %1 = icmp slt i64 %a, 5
+  %2 = trunc i64 %a to i32
+  %3 = select i1 %1, i32 %2, i32 5
+  ret i32 %3
+}
+
+; CHECK-LABEL: @t5
+; CHECK-NEXT: icmp
+; CHECK-NEXT: zext
+; CHECK-NEXT: select
+define i64 @t5(i32 %a) {
+  ; Same as @t3, but with mismatched signedness between icmp and zext.
+  ; InstCombine should leave this alone.
+  %1 = icmp slt i32 %a, 5
+  %2 = zext i32 %a to i64
+  %3 = select i1 %1, i64 5, i64 %2
+  ret i64 %3
+}
+
+; CHECK-LABEL: @t6
+; CHECK-NEXT: icmp
+; CHECK-NEXT: select
+; CHECK-NEXT: sitofp
+define float @t6(i32 %a) {
+  %1 = icmp slt i32 %a, 0
+  %2 = select i1 %1, i32 %a, i32 0
+  %3 = sitofp i32 %2 to float
+  ret float %3
+}