[PATCH] [AArch64] Multiply-Negate operation not lowered to mneg in AArch64

[Karthik Bhat]

Hi t.p.northover,

Hi All,
The below code-

int a,b,c;
void fn() {
  c = -(a*b);
}

when compiled on AArch64 bit with -O3 generates the assembly as follows-

fn:                                     // @fn
// BB#0:
	adrp	x8, a
	adrp	x9, b
	ldr	w8, [x8, :lo12:a]
	ldr	w9, [x9, :lo12:b]
	neg	 w8, w8
	mul	 w8, w9, w8
	adrp	x9, c
	str	w8, [x9, :lo12:c]
	ret

the neg and mul operation can be combined to mneg in AArch64. Gcc combines the above to mneg instruction.
Added a patch in llvm to do the same.

Post patch the following assembly is generated-
fn:                                     // @fn
// BB#0:
	adrp	x8, a
	adrp	x9, b
	ldr	w8, [x8, :lo12:a]
	ldr	w9, [x9, :lo12:b]
	mneg	 w8, w8, w9
	adrp	x9, c
	str	w8, [x9, :lo12:c]
	ret

Please let me know if this is good to commit.

Thanks and Regards
Karthik Bhat

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D6754

Files:
  lib/Target/AArch64/AArch64InstrInfo.td
  test/CodeGen/AArch64/dp-3source.ll

Index: lib/Target/AArch64/AArch64InstrInfo.td
===================================================================
--- lib/Target/AArch64/AArch64InstrInfo.td
+++ lib/Target/AArch64/AArch64InstrInfo.td
@@ -657,6 +657,10 @@
           (MSUBWrrr GPR32:$Rn, GPR32:$Rm, WZR)>;
 def : Pat<(i64 (ineg (mul GPR64:$Rn, GPR64:$Rm))),
           (MSUBXrrr GPR64:$Rn, GPR64:$Rm, XZR)>;
+def : Pat<(i32 (mul (ineg GPR32:$Rn), GPR32:$Rm)),
+          (MSUBWrrr GPR32:$Rn, GPR32:$Rm, WZR)>;
+def : Pat<(i64 (mul (ineg GPR64:$Rn), GPR64:$Rm)),
+          (MSUBXrrr GPR64:$Rn, GPR64:$Rm, XZR)>;
 } // AddedComplexity = 7
 
 let AddedComplexity = 5 in {
Index: test/CodeGen/AArch64/dp-3source.ll
===================================================================
--- test/CodeGen/AArch64/dp-3source.ll
+++ test/CodeGen/AArch64/dp-3source.ll
@@ -161,3 +161,18 @@
 ; CHECK: umnegl {{x[0-9]+}}, {{w[0-9]+}}, {{w[0-9]+}}
   ret i64 %res
 }
+
+ at a = common global i32 0, align 4
+ at b = common global i32 0, align 4
+ at c = common global i32 0, align 4
+
+define void @test_mneg(){
+; CHECK-LABEL: test_mneg:
+  %1 = load i32* @a, align 4
+  %2 = load i32* @b, align 4
+  %3 = sub i32 0, %1
+  %4 = mul i32 %2, %3
+  store i32 %4, i32* @c, align 4
+; CHECK: mneg {{w[0-9]+}}, {{w[0-9]+}}, {{w[0-9]+}}
+  ret void
+}

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