Subzero: Fix Phi lowering.

tests_lit/llvm2ice_tests/phi.ll

+; This tests some of the subtleties of Phi lowering.  In particular,
+; it tests that it does the right thing when it tries to enable
+; compare/branch fusing.
+
+; RUN: %llvm2ice -O2 --verbose none --no-phi-edge-split %s \
+; RUN:   | llvm-mc -triple=i686-none-nacl -x86-asm-syntax=intel -filetype=obj \
+; RUN:   | llvm-objdump -d -symbolize -x86-asm-syntax=intel - | FileCheck %s
+; RUN: %llvm2ice --verbose none %s | FileCheck --check-prefix=ERRORS %s
+; RUN: %llvm2iceinsts %s | %szdiff %s | FileCheck --check-prefix=DUMP %s
+; RUN: %llvm2iceinsts --pnacl %s | %szdiff %s \
+; RUN:                           | FileCheck --check-prefix=DUMP %s
+
+define internal i32 @testPhi1(i32 %arg) {
+entry:
+  %cmp1 = icmp sgt i32 %arg, 0
+  br i1 %cmp1, label %next, label %target
+next:
+  br label %target
+target:
+  %merge = phi i1 [ %cmp1, %entry ], [ false, %next ]
+  %result = zext i1 %merge to i32
+  ret i32 %result
+}
+; Test that compare/branch fusing does not happen, and Phi lowering is
+; put in the right place.
+; CHECK-LABEL: testPhi1
+; CHECK: cmp {{.*}}, 0
+; CHECK: mov {{.*}}, 1
+; CHECK: jg
+; CHECK: mov {{.*}}, 0
+; CHECK: mov [[PHI:.*]],
+; CHECK: cmp {{.*}}, 0

[jvoung (off chromium), 2014/09/10 17:17:43]

Would it be clearer to check for another jump? (that's what happens when fusing
doesn't happen right?)

[Jim Stichnoth, 2014/09/10 18:29:16]

The problem I had is that the current code generation looks like this:

	cmp	al, 0
	jne	.LtestPhi1$next
	jmp	.LtestPhi1$target
.LtestPhi1$next:
	mov	cl, 0
	jmp	.LtestPhi1$target
.LtestPhi1$target:

After we fix the excessive number of branches, it should look more like this:

	cmp	al, 0
	je	.LtestPhi1$target
	mov	cl, 0
.LtestPhi1$target:

So today's test for jne would break after this change.

However, after writing this up, I realize that my test for two labels will also
break, so yeah, let's make it clearer for now. :)

+; CHECK: :
+; CHECK: mov [[PHI]], 0
+; CHECK: :
+; CHECK: movzx {{.*}}, [[PHI]]
+
+define internal i32 @testPhi2(i32 %arg) {
+entry:
+  %cmp1 = icmp sgt i32 %arg, 0
+  br i1 %cmp1, label %next, label %target
+next:
+  br label %target
+target:
+  %merge = phi i32 [ %arg, %entry ], [ 0, %next ]

[jvoung (off chromium), 2014/09/10 17:17:43]

If you change the %arg to a recognizable constant, would that make it easier to
check for the assignment before the cmp?

[Jim Stichnoth, 2014/09/10 18:29:16]

Good idea, done.

+  ret i32 %merge
+}
+; Test that compare/branch fusing and Phi lowering happens as expected.
+; CHECK-LABEL: testPhi2
+; CHECK: cmp {{.*}}, 0
+; CHECK: jg

[jvoung (off chromium), 2014/09/10 17:17:43]

Should this be CHECK-NEXT?

[Jim Stichnoth, 2014/09/10 18:29:16]

Done.

+; CHECK: :
+; CHECK: mov [[PHI:.*]], 0
+; CHECK: :
+; CHECK: mov {{.*}}, [[PHI]]
+
+; ERRORS-NOT: ICE translation error
+; DUMP-NOT: SZ