[SLP] Fix sign-extends for type-shrinking

This patch ensures the correct minimum bit width during type-shrinking.
Previously when type-shrinking, we always sign-extended values back to their
original width. However, if we are going to sign-extend, and the sign bit is
unknown, we have to increase the minimum bit width by one bit so the
sign-extend will fill the upper bits correctly. If the sign bit is known to be
zero, we can perform a zero-extend instead. This should fix PR31243.

Reference: https://llvm.org/bugs/show_bug.cgi?id=31243
Differential Revision: https://reviews.llvm.org/D27466

llvm-svn: 289470

1 files changed, 72 insertions, 0 deletions

diff --git a/llvm/test/Transforms/SLPVectorizer/X86/minimum-sizes.ll b/llvm/test/Transforms/SLPVectorizer/X86/minimum-sizes.ll
new file mode 100644
index 00000000000..00af8ff4a2d
--- /dev/null
+++ b/llvm/test/Transforms/SLPVectorizer/X86/minimum-sizes.ll
@@ -0,0 +1,72 @@
+; RUN: opt -S -slp-threshold=-6 -slp-vectorizer -instcombine < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; These tests ensure that we do not regress due to PR31243. Note that we set
+; the SLP threshold to force vectorization even when not profitable.
+
+; CHECK-LABEL: @PR31243_zext
+;
+; When computing minimum sizes, if we can prove the sign bit is zero, we can
+; zero-extend the roots back to their original sizes.
+;
+; CHECK: %[[OR:.+]] = or <2 x i8> {{.*}}, <i8 1, i8 1>
+; CHECK: %[[E0:.+]] = extractelement <2 x i8> %[[OR]], i32 0
+; CHECK: %[[Z0:.+]] = zext i8 %[[E0]] to i64
+; CHECK: getelementptr inbounds i8, i8* %ptr, i64 %[[Z0]]
+; CHECK: %[[E1:.+]] = extractelement <2 x i8> %[[OR]], i32 1
+; CHECK: %[[Z1:.+]] = zext i8 %[[E1]] to i64
+; CHECK: getelementptr inbounds i8, i8* %ptr, i64 %[[Z1]]
+;
+define i8 @PR31243_zext(i8 %v0, i8 %v1, i8 %v2, i8 %v3, i8* %ptr) {
+entry:
+  %tmp0 = zext i8 %v0 to i32
+  %tmp1 = zext i8 %v1 to i32
+  %tmp2 = or i32 %tmp0, 1
+  %tmp3 = or i32 %tmp1, 1
+  %tmp4 = getelementptr inbounds i8, i8* %ptr, i32 %tmp2
+  %tmp5 = getelementptr inbounds i8, i8* %ptr, i32 %tmp3
+  %tmp6 = load i8, i8* %tmp4
+  %tmp7 = load i8, i8* %tmp5
+  %tmp8 = add i8 %tmp6, %tmp7
+  ret i8 %tmp8
+}
+
+; CHECK-LABEL: @PR31243_sext
+;
+; When computing minimum sizes, if we cannot prove the sign bit is zero, we
+; have to include one extra bit for signedness since we will sign-extend the
+; roots.
+;
+; FIXME: This test is suboptimal since the compuation can be performed in i8.
+;        In general, we need to add an extra bit to the maximum bit width only
+;        if we can't prove that the upper bit of the original type is equal to
+;        the upper bit of the proposed smaller type. If these two bits are the
+;        same (either zero or one) we know that sign-extending from the smaller
+;        type will result in the same value. Since we don't yet perform this
+;        optimization, we make the proposed smaller type (i8) larger (i16) to
+;        ensure correctness.
+;
+; CHECK: %[[S0:.+]] = sext <2 x i8> {{.*}} to <2 x i16>
+; CHECK: %[[OR:.+]] = or <2 x i16> %[[S0]], <i16 1, i16 1>
+; CHECK: %[[E0:.+]] = extractelement <2 x i16> %[[OR]], i32 0
+; CHECK: %[[S1:.+]] = sext i16 %[[E0]] to i64
+; CHECK: getelementptr inbounds i8, i8* %ptr, i64 %[[S1]]
+; CHECK: %[[E1:.+]] = extractelement <2 x i16> %[[OR]], i32 1
+; CHECK: %[[S2:.+]] = sext i16 %[[E1]] to i64
+; CHECK: getelementptr inbounds i8, i8* %ptr, i64 %[[S2]]
+;
+define i8 @PR31243_sext(i8 %v0, i8 %v1, i8 %v2, i8 %v3, i8* %ptr) {
+entry:
+  %tmp0 = sext i8 %v0 to i32
+  %tmp1 = sext i8 %v1 to i32
+  %tmp2 = or i32 %tmp0, 1
+  %tmp3 = or i32 %tmp1, 1
+  %tmp4 = getelementptr inbounds i8, i8* %ptr, i32 %tmp2
+  %tmp5 = getelementptr inbounds i8, i8* %ptr, i32 %tmp3
+  %tmp6 = load i8, i8* %tmp4
+  %tmp7 = load i8, i8* %tmp5
+  %tmp8 = add i8 %tmp6, %tmp7
+  ret i8 %tmp8
+}