[GlobalISel] Combine (0 * x) -> 0

[paquette]

Summary

Before, we only folded (x * 0) -> 0. It's possible that the 0 can show up on the RHS instead of the LHS. (E.g. inlining a memset and getting a multiply of two constants.)

So, let's fold 0 on the LHS as well.

Example: https://godbolt.org/z/CHQVs9

[arsenm]

We're missing the canonicalizations in MIR. It would be better to fix that instead?

[aemerson]

LGTM but I think this is unnecessary since it was a simple fix to just avoid generating useless multiplies in fc905ae003df

Feel free to commit anyway.

[paquette]

I think I'd rather take the canonicalization route. I'll drop this so we can do this the right way.

llvm/include/llvm/Target/GlobalISel/Combine.td

def binop_same_val: GICombineRule<
   (match (wip_match_opcode G_AND, G_OR):$root,
     [{ return Helper.matchBinOpSameVal(*${root}); }]),
   (apply [{ return Helper.replaceSingleDefInstWithOperand(*${root}, 1); }])
 >;
 
 // Fold (0 op x) - > 0
 def binop_left_to_zero: GICombineRule<
   (defs root:$root),
-  (match (wip_match_opcode G_SDIV, G_UDIV, G_SREM, G_UREM):$root,
+  (match (wip_match_opcode G_SDIV, G_UDIV, G_SREM, G_UREM, G_MUL):$root,
     [{ return Helper.matchOperandIsZero(*${root}, 1); }]),
   (apply [{ return Helper.replaceSingleDefInstWithOperand(*${root}, 1); }])
 >;
 
 // Fold (x op 0) - > 0
 def binop_right_to_zero: GICombineRule<
   (defs root:$root),
   (match (wip_match_opcode G_MUL):$root,

llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-trivial-arith.mir

bb.1.entry:
     %x:_(s32) = COPY $w0
     %cst:_(s32) = G_CONSTANT i32 0
     %op:_(s32) = G_ADD %x(s32), %cst
     $w0 = COPY %op(s32)
     RET_ReallyLR implicit $w0
 
 ...
 ---
-name:            mul_0
+name:            mul_0_rhs
 tracksRegLiveness: true
 body:             |
   bb.1.entry:
     liveins: $w0
     ; Fold (x * 0) -> 0
     ;
-    ; CHECK-LABEL: name: mul_0
+    ; CHECK-LABEL: name: mul_0_rhs
     ; CHECK: liveins: $w0
     ; CHECK: %cst:_(s32) = G_CONSTANT i32 0
     ; CHECK: $w0 = COPY %cst(s32)
     ; CHECK: RET_ReallyLR implicit $w0
     %x:_(s32) = COPY $w0
     %cst:_(s32) = G_CONSTANT i32 0
     %op:_(s32) = G_MUL %x(s32), %cst
     $w0 = COPY %op(s32)
     RET_ReallyLR implicit $w0
 
 ...
+---
+name:            mul_0_lhs
+tracksRegLiveness: true
+body:             |
+  bb.1.entry:
+    liveins: $w0
+    ; Fold (0 * x) -> 0
+    ;
+    ; CHECK-LABEL: name: mul_0_lhs
+    ; CHECK: liveins: $w0
+    ; CHECK: %cst:_(s32) = G_CONSTANT i32 0
+    ; CHECK: $w0 = COPY %cst(s32)
+    ; CHECK: RET_ReallyLR implicit $w0
+    %x:_(s32) = COPY $w0
+    %cst:_(s32) = G_CONSTANT i32 0
+    %op:_(s32) = G_MUL %cst(s32), %x
+    $w0 = COPY %op(s32)
+    RET_ReallyLR implicit $w0
+
+...
 
 # FIXME: Probably should be able to replace this.
 ---
 name:            mul_0_cant_replace
 tracksRegLiveness: true
 body:             |
   bb.1.entry:
     liveins: $w0