ac/llvm: add option to clamp division by zero

Replace div(x) by min(div(x), FLT_MAX)) to avoid getting a NaN result
when x is 0.

A cheaper alternative would be to use legacy mult instructions but they're
not exposed by LLVM.

Cc: mesa-stable
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6259>
(cherry picked from commit 32f46a55c8)

.pick_status.json

@@ -3262,7 +3262,7 @@
         "description": "ac/llvm: add option to clamp division by zero",
         "nominated": true,
         "nomination_type": 0,
-        "resolution": 0,
+        "resolution": 1,
         "master_sha": null,
         "because_sha": null
     },

src/amd/llvm/ac_nir_to_llvm.c

@@ -713,6 +713,9 @@ static void visit_alu(struct ac_nir_context *ctx, const nir_alu_instr *instr)
 			result = emit_intrin_1f_param(&ctx->ac, "llvm.amdgcn.rcp",
 						      ac_to_float_type(&ctx->ac, def_type), src[0]);
 		}
+		if (ctx->abi->clamp_div_by_zero)
+			result = ac_build_fmin(&ctx->ac, result,
+					       LLVMConstReal(ac_to_float_type(&ctx->ac, def_type), FLT_MAX));
 		break;
 	case nir_op_iand:
 		result = LLVMBuildAnd(ctx->ac.builder, src[0], src[1], "");
@@ -859,6 +862,9 @@ static void visit_alu(struct ac_nir_context *ctx, const nir_alu_instr *instr)
 	case nir_op_frsq:
 		result = emit_intrin_1f_param(&ctx->ac, "llvm.amdgcn.rsq",
 					      ac_to_float_type(&ctx->ac, def_type), src[0]);
+		if (ctx->abi->clamp_div_by_zero)
+			result = ac_build_fmin(&ctx->ac, result,
+					       LLVMConstReal(ac_to_float_type(&ctx->ac, def_type), FLT_MAX));
 		break;
 	case nir_op_frexp_exp:
 		src[0] = ac_to_float(&ctx->ac, src[0]);
@@ -900,7 +906,7 @@ static void visit_alu(struct ac_nir_context *ctx, const nir_alu_instr *instr)
 	case nir_op_ffma:
 		/* FMA is better on GFX10, because it has FMA units instead of MUL-ADD units. */
 		result = emit_intrin_3f_param(&ctx->ac, ctx->ac.chip_class >= GFX10 ? "llvm.fma" : "llvm.fmuladd",
-		                              ac_to_float_type(&ctx->ac, def_type), src[0], src[1], src[2]);
+					      ac_to_float_type(&ctx->ac, def_type), src[0], src[1], src[2]);
 		break;
 	case nir_op_ldexp:
 		src[0] = ac_to_float(&ctx->ac, src[0]);

src/amd/llvm/ac_shader_abi.h

@@ -192,6 +192,9 @@ struct ac_shader_abi {
 
 	/* Whether undef values must be converted to zero */
 	bool convert_undef_to_zero;
+
+	/* Clamp div by 0 (so it won't produce NaN) */
+	bool clamp_div_by_zero;
 };
 
 #endif /* AC_SHADER_ABI_H */