InstructionSimplify: don't speculate about Constants changing.

When presented with an icmp/select pair, we can end up asking what would happen
if we replaced one constant with another in an instruction. This is a mistake,
while non-constant Values could become a constant, constants cannot change and
trying to do so can lead to completely invalid IR (a GEP referencing a
non-existant field in the original case).

llvm-svn: 303580

1 files changed, 39 insertions, 0 deletions

diff --git a/llvm/test/Transforms/EarlyCSE/const-speculation.ll b/llvm/test/Transforms/EarlyCSE/const-speculation.ll
new file mode 100644
index 00000000000..5b7f2f5b698
--- /dev/null
+++ b/llvm/test/Transforms/EarlyCSE/const-speculation.ll
@@ -0,0 +1,39 @@
+; RUN: opt -early-cse -S %s | FileCheck %s
+
+%mystruct = type { i32 }
+
+; @var is global so that *every* GEP argument is Constant.
+@var = external global %mystruct
+
+; Control flow is to make the dominance tree consider the final icmp before it
+; gets to simplify the purely constant one (%tst). Since that icmp uses the
+; select that gets considered next. Finally the select simplification looks at
+; the %tst icmp and we don't want it to speculate about what happens if "i32 0"
+; is actually "i32 1", broken universes are automatic UB.
+;
+; In this case doing the speculation would create an invalid GEP(@var, 0, 1) and
+; crash.
+
+define i1 @test_constant_speculation() {
+; CHECK-LABEL: define i1 @test_constant_speculation
+entry:
+  br i1 undef, label %end, label %select
+
+select:
+; CHECK: select:
+; CHECK-NOT: icmp
+; CHECK-NOT: getelementptr
+; CHECK-NOT: select
+
+  %tst = icmp eq i32 1, 0
+  %elt = getelementptr %mystruct, %mystruct* @var, i64 0, i32 0
+  %sel = select i1 %tst, i32* null, i32* %elt
+  br label %end
+
+end:
+; CHECK: end:
+; CHECK: %tmp = phi i32* [ null, %entry ], [ getelementptr inbounds (%mystruct, %mystruct* @var, i64 0, i32 0), %select ]
+  %tmp = phi i32* [null, %entry], [%sel, %select]
+  %res = icmp eq i32* %tmp, null
+  ret i1 %res
+}