[SCCP] Teach the pass how to handle `div` with overdefined operands.

This can prove that:

extern int f;
int g() {
    int x = 0;
    for (int i = 0; i < 365; ++i) {
        x /= f;
    }
    return x;
}

always returns zero. Thanks to Sanjoy for confirming this
transformation actually made sense (bugs are mine).

llvm-svn: 292531

1 files changed, 32 insertions, 0 deletions

diff --git a/llvm/test/Transforms/SCCP/overdefined-div.ll b/llvm/test/Transforms/SCCP/overdefined-div.ll
new file mode 100644
index 00000000000..f0b16155c17
--- /dev/null
+++ b/llvm/test/Transforms/SCCP/overdefined-div.ll
@@ -0,0 +1,32 @@
+; RUN: opt < %s -sccp -S | FileCheck %s
+
+; Test that SCCP has basic knowledge of when div can nuke overdefined values.
+
+; 0 / X = 0 even if X is overdefined.
+; CHECK-LABEL: test1
+; CHECK-NEXT: ret i32 0
+define i32 @test1(i32 %foo) {
+  %tinkywinky = udiv i32 0, %foo
+  ret i32 %tinkywinky
+}
+
+; CHECK-LABEL: test2
+; CHECK-NEXT: ret i32 0
+define i32 @test2(i32 %foo) {
+  %tinkywinky = sdiv i32 0, %foo
+  ret i32 %tinkywinky
+}
+
+; CHECK-LABEL: test3
+; CHECK: ret i32 %tinkywinky
+define i32 @test3(i32 %foo) {
+  %tinkywinky = udiv i32 %foo, 0
+  ret i32 %tinkywinky
+}
+
+; CHECK-LABEL: test4
+; CHECK: ret i32 %tinkywinky
+define i32 @test4(i32 %foo) {
+  %tinkywinky = sdiv i32 %foo, 0
+  ret i32 %tinkywinky
+}