Fix PR2506 by being a bit more careful about reverse fact propagation when

disproving a condition.  This actually compiles the existing testcase
(udiv_select_to_select_shift) to:

define i64 @test(i64 %X, i1 %Cond) {
entry:
	%divisor1.t = lshr i64 %X, 3		; <i64> [#uses=1]
	%quotient2 = lshr i64 %X, 3		; <i64> [#uses=1]
	%sum = add i64 %divisor1.t, %quotient2		; <i64> [#uses=1]
	ret i64 %sum
}

instead of:

define i64 @test(i64 %X, i1 %Cond) {
entry:
	%quotient1.v = select i1 %Cond, i64 3, i64 4		; <i64> [#uses=1]
	%quotient1 = lshr i64 %X, %quotient1.v		; <i64> [#uses=1]
	%quotient2 = lshr i64 %X, 3		; <i64> [#uses=1]
	%sum = add i64 %quotient1, %quotient2		; <i64> [#uses=1]
	ret i64 %sum
}

llvm-svn: 53534

2 files changed, 19 insertions, 3 deletions

diff --git a/llvm/test/Transforms/InstCombine/2008-07-13-DivZero.ll b/llvm/test/Transforms/InstCombine/2008-07-13-DivZero.ll
new file mode 100644
index 00000000000..85c3dbc5ff6
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/2008-07-13-DivZero.ll
@@ -0,0 +1,16 @@
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep {lshr.*3}
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep {call .*%cond}
+; PR2506
+
+; We can simplify the operand of udiv to '8', but not the operand to the
+; call.  If the callee never returns, we can't assume the div is reachable.
+define i32 @a(i32 %x, i32 %y) {
+entry:
+        %tobool = icmp ne i32 %y, 0             ; <i1> [#uses=1]
+        %cond = select i1 %tobool, i32 8, i32 0         ; <i32> [#uses=2]
+        %call = call i32 @b( i32 %cond )                ; <i32> [#uses=0]
+        %div = udiv i32 %x, %cond               ; <i32> [#uses=1]
+        ret i32 %div
+}
+
+declare i32 @b(i32)
diff --git a/llvm/test/Transforms/InstCombine/udiv_select_to_select_shift.ll b/llvm/test/Transforms/InstCombine/udiv_select_to_select_shift.ll
index 277592c983d..614ae3dc975 100644
--- a/llvm/test/Transforms/InstCombine/udiv_select_to_select_shift.ll
+++ b/llvm/test/Transforms/InstCombine/udiv_select_to_select_shift.ll
@@ -2,13 +2,13 @@
 ; udiv X, (Select Cond, C1, C2) --> Select Cond, (shr X, C1), (shr X, C2)
 ;
 ; RUN: llvm-as < %s | opt -instcombine | llvm-dis -f -o %t
-; RUN:   grep select %t | count 1
+; RUN:   not grep select %t
 ; RUN:   grep lshr %t | count 2
-; RUN:   ignore grep udiv %t | count 0
+; RUN:   not grep udiv %t
 
 define i64 @test(i64 %X, i1 %Cond ) {
 entry:
-        %divisor1 = select i1 %Cond, i64 8, i64 16
+        %divisor1 = select i1 %Cond, i64 16, i64 8
         %quotient1 = udiv i64 %X, %divisor1
         %divisor2 = select i1 %Cond, i64 8, i64 0
         %quotient2 = udiv i64 %X, %divisor2