[SCCP] Teach the pass about `mul %x 0` even if %x is overdefined.

The motivating example is:

extern int patatino;
int goo() {
    int x = 0;
    for (int i = 0; i < 1000000; ++i) {
        x *= patatino;
    }
    return x;
}

Currently SCCP will not realize that this function returns always zero,
therefore will try to unroll and vectorize the loop at -O3 producing an
awful lot of (useless) code. With this change, it will just produce:

0000000000000000 <g>:
   xor    %eax,%eax
   retq

llvm-svn: 289175

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/test/Transforms/SCCP/logical-nuke.ll b/llvm/test/Transforms/SCCP/logical-nuke.ll
index 4ef52a25539..76884dbab4b 100644
--- a/llvm/test/Transforms/SCCP/logical-nuke.ll
+++ b/llvm/test/Transforms/SCCP/logical-nuke.ll
@@ -1,6 +1,6 @@
 ; RUN: opt < %s -sccp -S | FileCheck %s
 
-; Test that SCCP has basic knowledge of when and/or nuke overdefined values.
+; Test that SCCP has basic knowledge of when and/or/mul nuke overdefined values.
 
 ; CHECK-LABEL: test
 ; CHECK: ret i32 0
@@ -29,3 +29,10 @@ define i32 @test4(i32 %X) {
   %Y = or i32 %X, undef
   ret i32 %Y
 }
+
+; CHECK-LABEL: test5
+; CHECK: ret i32 0
+define i32 @test5(i32 %foo) {
+  %patatino = mul i32 %foo, undef
+  ret i32 %patatino
+}